It seems that the "killer feature" of XCode 4.3, the new Storyboard editor, doesn't like VNC. An important feature of this is the ability to hold down control and then just drag objects around to form associations, but over VNC, nothing happens. It's also not possible to move objects around on the storyboard.

I tried this with Lion's "Screen Sharing", RealVNC and Vine Server, all with the same problem. Also tried it with several different VNC clients, all with the same result. I don't have an answer for it except to wait for an update to XCode, but at least knowing that it's not just you might help.

I have to admit that I did give in and buy a Startech KVM...