In my experience as a health insurance broker, here's how it works. If you have a lapse in coverage longer than 60 days between jobs, that's the only time you can get pre-exed. And only for the treatment of that particular illness for 12 months. After 12 months of paying in, your covered no matter what you have assuming you've lapsed. It's based on the theory that you will get another job within 60 days of a lapse. The key is if coverage has officially lapsed, but you have options to continue coverage either through COBRA or buying an individual policy that falls under HIPPA. Those are peoples options if they have the money to float their premiums between jobs. It's definitely a motivator to stay working.
No pre-ex for Rx's you take that treat the illness and for a few other things like pregnancy.
It applies to group major med plans, so it depends if your employer offers benefits and what policies they have. This is for workers. If you're too old to work, you get medicare. Everybody else, medicaid and other state specific plans. It's very involved depending on individual circumstances, but that's it in a nutshell.
Addendum, HIPPA applies to most group health insurance plans if an employer offers one. Employers have tax incentives to offer plans but it's still not cheap. An employers key motivators in offering benefits plans is recruiting and retention of good employees.
Like I said, it's complicated...so that's why it takes getting licensed with mandated continuing education every two years to even sell insurance. Health insurance is one of the most heavily regulated industries so it's why you have brokers, agents, and HR Departments.
Assuming ObamaCare passes, HR Departments will get alot bigger, but the brokers and agents will shrink. I'm getting ready to get legislated out of business most likely.