I agree with what everyone here has said. I'll add a couple of thoughts I had. First is that you could work for an organization like the ACLU or a private firm specializing in constitutional litigation. These firms may be very selective, and the ACLU might not be to your taste, but they do a lot of Section 1983 litigation (suing the gov't for violations of const'l rights). From the exposure to it I've had, that kind of litigation seems very complex, with a lot of interrelated issues. That could be something to look into.
Second, you could go into academia, although this is infinitely more difficult, as you'll need the proper school pedigree, a great clerkship, etc. One of the best professors I've ever had was for Federal Courts, which is loaded with federalism and sep of powers issues. He went to Yale and was a SCOTUS clerk. He gets to think and write about these issues every day.
You don't necessarily need to go to Yale and clerk for a Supreme Court Justice to get into academia, but it can still be a difficult route. Could be something to work towards?
This is my point, I suppose. There are separation of powers and federalism issues in public policy all the time. There have to be lawsuits involving them. And, in order to do lawsuits, some one has to litigate.
I want to be one of those guys. I'm going to a great school (not Yale though), so I'm just curious how I can plug myself into that niche. I'll look into those small firms you were talking about.
Thanks for your help guys. If there are any more thoughts, I'd be grateful to hear them.