I would say as long as you haven't had any complications from donation, kidney donation (that is, having one kidney) should not be considered a pre-existing condition that makes the donor uninsurable. But we don't have the force of law behind us, just the understanding of health insurance underwriters.
The health care reform law outlawed pre-existing condition exclusions for children under age 19. That rule generally took effect at the beginning of this year. The provision outlawing pre-existing condition exclusions for everyone, including adults, doesn't take effect until 2014. Whether that rule (and many other aspects of the reform law) actually takes effect will likely depend on the outcome of next year's elections.
Whether getting insurance will be an issue depends on a lot of factors (your general health, individual vs. group insurance, health of other covered family members, etc.), so it's difficult to generalize. My experience is that kidney donation for the most part has NOT been a barrier to getting medical insurance. If it happens, let us know, and we'll do what we can to act as your advocate.