You can pretty safely move jobs and group plans even when you have a pre-existing condition, provided there's no significant break in coverage. It's part of the "portability" portion of HIPAA.
Wow. I didn't know that. I yield all my comment's karma to the man whose comment is actually correct.
I think I missed this because portability doesn't guarantee issuance of individual (non-group) health insurance. It's a very good thing for people who change jobs, but not a very good thing for people who want to start companies.