I'm pretty much exactly like you (second year Physics and Math, and was thinking about switching to engineering) so I've thought about this a lot myself. The conclusion I came to was that job prospects for Physics (or math) PhD's are not bad at all, especially if you're willing to relocate to NY and work in finance and I think a very experienced person on this forum (twofish-quant) would agree with me on that. According to him starting salaries for a Physics PhD's working in finance in NY can be upto 130k and within a few years cross the 200k mark. Furthermore, if you can gain a significant amount of programming experience while doing your PhD, jobs as a programmer aren't hard to find either. From what I've heard jobs are poor in academia though, so as long as you're willing to leave academia, I think you should be fine with a Physics PhD.
After you are done with your bachelors and you decide that a PhD isn't for you, you can always go for a masters in engineering and then work as an engineer from there on. So my current plan is to continue double majoring in Math and Physics, maybe minor in an engineering discipline that I may find interesting, and if I don't feel like doing a PhD anymore, get a masters in engineering. Otherwise, I'm going to get a PhD and hopefully it should be fine job-wise.
Edit: For those finance jobs significant programming experience (especially in C++) is highly desirable, right twofish?