I am a transplant patient. I have had a liver-kidney transplant as the culmination of a genetic condition. I have, obviously, had the condition my whole life. When I got out of college into a job with medical insurance, there were no "pre-existing condition" questions at all. As a salaried employee, I was immediately eligible for benefits on day 1. I was told this by an HR rep, and so it proved to be.
When I got the transplant (years later), I did get a letter from the insurance company - but they were checking to see if there was someone they could sue for the expense (there wasn't). A little seedy, but not refusing to pay.
The stories I've heard of insurance companies refusing to pay after years generally have 2 criteria: Not part of a group plan and failing to disclose a condition upon initial application. The deal is, if they give you a questionaire, the rates are dependent on your answers. So, if you say your perfectly healthy, you get a lower rate. If they then find out that you weren't perfectly healthy, and you knew that, then you're going to be in trouble. Group plans typically don't have the questionaire - I've only ever been asked if I smoke, for example. If I did smoke, and lied about it to get a lower rate, I should expect that to catch up to me when I get lung cancer.