Actually, an important point needs to be emphasized here. A field may be "hot" at any given moment, but it doesn't mean that there's a lot of job opportunities in that particular field. HEP is a prime example. With the LHC in the news, a lot of people think it is a "hot" field, but look at the employment rate in that field. In fact, if you are in the US, chances are you are abandoning that field because the US budget has systematically cut DOE HEP budget for the past several years!
The degree of "hotness" of a field often depends on the funding profile. In the US, the DOE, with budget approval, can often steer the direction of research, such as via establishing the "Science and Technology Centers". This was done in the early 90's for superconductivity, and then later on done for Nanoscience. But these are not easy to predict, and it all depends on what has been discovered, what is considered to be of national importance at the moment, etc. Research on hybrid/electric vehicles, batteries, etc. is "hot" right now because of increase funding, all due to the high cost of oil. But now that oil has been dropping like mad, one can easily foresee the directed funding to go in another direction. Who would have predicted that?
Again, look at the history of science, and the history of funding, and you tell me if any of you saw these things coming way back then. If you did, then maybe I'll give my attention to you when you make such predictions.
Zz.