If you really want the answer to this question, take it to the radiology forum for the inside scoop. I swore I was done with this thread, but since you asked...
I do know that the radiologists face some trouble due to the fact that with tele-radiology, it's getting much easier to outsource their services to cheaper radiologists overseas. My guess is that if you're good at what you do, or have a fellowship, you'll be okay, but ask some radiologists.
Ectopic Fetus is right that there will continue to be downward pressure on all physicians salaries. The more you make, the more they can cut. We do have the ultimate trump card on that, though. The more docs that leave certain specialties, leave Medicine in general, retire early, cut their hours, or just plain don't go into Medicine, the more incentive there is to pay docs more, give better benefits, better hours, better working conditions and less ridiculous administrative interference. The government could counter this, however, by letting more foreign medical grads in the country, who generally come from countries where doctors make much less, and therefore are willing to work for less (yes, even you and I can be outsourced). If the American people are okay with that, that makes things harder for us.
In my opinion, since the large top-end financial rewards likely won't be there, the premium will more and more be on the jobs/specialties where you can have the least stress, the most normal hours (8-4 Mon-Fri), best lifestyle and least night shifts and night-call for emergencies.
That being said, if you pick something you enjoy (and will always enjoy), the rest will likely work itself out. Also, the more unique your niche (specialty + fellowship, instead of 3 year primary care residency) the more insulated you'll be from all of this uncertainty.