I have no plans to upgrade to Leopard any time soon, so there isn't going to be anything useful from me on this. I do have updated project files for Xcode 2, but that is only for my Xt tree, so it's totally useless for general CVS usage until I'm eventually able to commit all of my changes.
Someone is just going to have to pony up an Xcode 3.0 project file for us. I can commit it to CVS, but I can't create it myself.
Making it compatible with earlier OS X version is generally pretty trivial though (assuming that you don't need to change compiler versions, like with supporting early 10.3 versions). The Xcode docs have plenty of good information to do what you need, and it shouldn't take more than a half-hour at most to figure out whatever you need on the subject. I remember that it took me longer to actually compile the builds than it did to figure out how to even do it.