Rescinding earlier comments...
I'm not going to put any more effort into trying to solve the "framework compatibility" problem that was discussed, because after talking with some people over at MS (thanks RobH and Scott Louveau) I've decided to stand by my original statement. The only responsible thing that control developers should do is have a build for each version of the Framework. I don't want my 1.1-compiled app trying to run on 1.0, and you shouldn't either. You want to give your users the best experience possible, and by only providing one version, you are not doing so. I will blog about a few possible solutions that MS suggests within the next day or so, but I agree with them and their recommendations.