As Clark/John points out, there is a government-operated insurance plan available for people with pre-existing conditions. That program runs until 2014, when the state-run insurance exchanges are supposed to be available. The law also changes at that time, preventing insurance companies from excluding anyone with a pre-existing condition. All of the foregoing assumes the current health care reform law remains intact.
LDO has been contacted in the past about people having difficulty getting insurance after donating, but the number of incidences I've been able to confirm have been very few. Most insurance company underwriters are enlightened enough to know that donating--in and of itself--is not a health condition. I don't know of any situations where a donor was dropped from existing coverage by an insurance carrier.
Do what you can to keep the coverage you have (pay those premiums on time!), and you should be fine. I doubt your carrier would be able to drop you because you donated. If they do, please contact us and we'll be your advocate to get coverage restored.