Twofish-quant's advice is excellent, and I'd like to add a few thoughts from the perspective of a trader-quant for what they are worth. There are many different roles for physicists in finance. In this sense, wall street is similar to academia. Your individual experience, career development, and pursuits are highly dependent on the specific group in which you work. Though there is mobility between firms, to a variable degree within a firm, and to a rapidly diminishing degree between niches, the initial step you take likely will determine the course of you career. So make it carefully.
There are several types of firms and roles for quants. Large firms such as the major investment banks are good starting points. They have the pluses and minuses of a postdoc at a top tier university. They look great on the resume and have many exciting areas to work in and high quality people to learn from, but you probably won't end up there permanently. Many people start at these firms on the "sell-side", learn a lot, and then move to some sort of "buy-side" firm -- either the prop branch of an IB or a hedge fund. Hedge funds can be great places to work, though I tend not to recommend them as a first job. Often the starting compensation is higher, and the environment can be cozier for a quiet academic. Generally, your boss is your entire world there -- so choose carefully if you follow this route. Also, they have traditionally been considered career destinations rather than entry points. Be mindful of whether the hedge fund is large and well known. Only a few of the biggest, best will help your resume.
Some common roles for physicists are listed below. Bear in mind that preparation is difficult without knowing your ultimate path. Programming experience in C++ and/or Java is important, as is knowledge of at least one scripting language (Python, Perl, etc). Knowing your way around a *nix environment can't hurt either. Be careful how you portray yourself, however. Unless you want to become a programmer, avoid pegging yourself that way.
(1) Developer/Programmer: If you enjoy programming, this can be a great route. Large institutions can be challenging for the starting physicist, and long term career growth tends to be along the managerial route. My personal opinion is that small firms are much better in this regard, even if the risks are higher, because you probably don't really know what you want to do. Despite copious claims to the contrary, my observation has been that it is very hard to move from I.T. or other purely programming roles into research or trading positions. If you are targetting such roles, expect to be grilled hard. Re-reading Stroustrup's book on C++ isn't a bad idea. Some additional topics people tend to ask about: STL, Boost, Template Metaprogramming, various simple containers and algorithms.
(2) Statistical Arbitrage: If you want to play with statistics and write programs to beat the markets this is a good area. It combines lots of programming with statistical inference and computer theory. This can be an excellent field for an experimental particle physicist such as yourself. The data issues and statistical analysis are quite similar. If you're lucky, a role in statistical arbitrage can combine research and trading.
(3) Options theory/interest rate modeling (MBS, etc): This uses path integrals, Monte carlo analysis, etc to integrate over paths. There are elements of stochastic analysis, etc, as well. If you like heavy duty math and aren't too worried about statistical validity, this is a very interesting field.
I would suggest that you not apply through HR (firmwide or I.T.) at most large firms. As in academia, you are much better off contacting a specific group that is of interest. Ask for an informational interview with the head of the group. If you make a good impression, they either will consider you for their group or pass you along to colleagues. Despite the apparent competition in this job market, there is a matching inefficiency and good candidates are scarce. Almost all my hires have been referred to me.
My best suggestion is that you allocate a few months to shamelessly networking, determine the specific opportunities available to you and whether they are of interest, and make a decision by a fixed date. Assume that you will remain in your first job for at least 3 years and ignore any promises about your career path. It's great if they transpire, but things change quickly on wall street and you can't count on them. Also, don't worry about the money once you start. The first few years are for learning and building a reputation; think of them as a postdoc. All of the quants I have know who stuck it out for 8+ years were happy financially (though as twofish-quant points out, it is relative). The one thing you cannot afford to have on Wall Street is an ego. If you compare yourself to the next guy, then you will always be unhappy. The worst career blunders I have seen have arisen from that sort of thinking. Otherwise, the $$$ can be quite nice.
On the practical points:
Hours: This depends on your goal. If you want to take the fast track (or have a good shot at it), you will have to work very long hours. This should be fun, though. If not, the slow track is fine. You will do well financially and can find the balance you desire. If you can afford to, I would suggest committing yourself 100% the first few years and then things should ease up a bit (and you'll have a sense of what you want).
Social life: Same as anywhere. Others are in the same boat, and the city abounds with ways to accommodate young people with money who want to meet. Starting a family can be rough if both of you are working. After a couple of years, when you know your way around the city, you would have an easier time. I've known many people who've done this and are quite happy with their lives.
Location: NY, London, Tokyo. There are some hedge funds in Chicago, Greenwich/Stamford, and California, but most likely you will spend your career in one of those 3 cities. There are worse fates as these things go.
Money: You will be poor the first year. My standard of living during my first year on Wall street was lower than as a grad student. After a year or two most people seem to do ok. Bear in mind that pay can be high on wall street, but there is no job stability. That's the tradeoff. This is something to consider if you one day wish to raise a family.
Internship: This can help, and may give you a sense of whether a firm, group, or field will work for you. If you are well received, there is a chance they may try to buy you out of finishing grad school. Approach this as you would a job -- target individual groups if you can.
Pros: Money. Can be challenging and exciting. There is a satisfaction to being in the middle of the action.
Cons: It's not physics.
I hope this helps. Best of luck.
Cheers,
Osmosis