Regulations vary by state, but in general you cannot be denied coverage under a group plan
(like you would get through an employer) for a pre-existing condition. This was part of the federal HIPAA that was passed during the late 90's. (Now, for individual plans, it's a different story, though the new health care law was supposed to address that.)
The caveat is that you have to be able to show proof of continuous coverage. If you're jumping from one employer to the next, this usually isn't a problem. But, if you get laid off or something, that's why it is a good idea to continue COBRA coverage for as long as possible (I know this can be very expensive). If there is a gap in coverage, there's some sort of grace period before the new plan has to cover the pre-existing condition. This is one of the things that varies by state, but the grace period length is usually based on how long you went without coverage.
This might make your head swim, but here's some info on the HIPAA:
I don't know if or how the current legislation affects this, but most of those regulations don't go immediately into effect anyway.