I found this while aimlessly poking around the 'net, thought it was pretty cool. I can't attest to the accuracy of it, but I'm inclined to believe it's fairly trustworthy. Fun to look around in, if nothing else.
North American Plant Atlas
EDIT: Really, don't rely on the accuracy of this because, through further poking around by other members, it does not appear to be entirely accurate. It might be more accurate for the non-CP species, but I wouldn't know.