It is state specific. I can only comment about California. Here the state regulations prohibit exclusions for over 6 months , so all of the carriers either accept you or decline. Usually they decline.
They can set a very high rate surcharge for someone , overweight , smoker ,etc, as long as the surcharge is applied to everyone in that policy line equally . You can only be canceled for lapse of payment, fraud , or the carrier discontuning that specific line of policy.You cannot be canceled or have your rate raised because of claims.
The application will ask about any injury , illness, disease that you have received treatment for, sought treatment for, or even had a health professional advised you to seek treatment for,or should have sought treatment for, during the last ten years. The insurrance co. wants to cover their a$$ on everything!
The way it is in CA, Anyone over 25 is declined unless in super good health , with almost no illness needing treatment or meds for the prev. 10 years. Ya, it sucks.
If you have group coverage, C.O.B.R.A. , then conversion is how most get coverage when leaving the group.
The state runs a high risk pool , very high rates, crapy coverage, limited term, ( 2 years max I think), and the max cap on benefits is $80k I think. To qualify, you must prove 2 declines, and no coverage for 6 mo.
Do a search on this site if you have not already done so. You will find dozens of threads on the subject.
The Healthcare Reform act will change the pre-existing exclusions , but except for children, that part does not kick in till 2014.