Mac OS X 10.5 introduces some cool things, for example the new for(each)-loop in Objective C 2.0 and some other handy improvements in Xcode 3.0 (controls, CoreImage-rendering, etc). But I think they are for 10.5+ only? Can one create backwards compatible projects in Xcode 3.0 when using the new Interface Builder 3.0? Or do we have to wait until Leopard spread widely enough to release/write software for it?!
As always, you can use the new IDE to create software for old operating systems, but if you use new features such as these then you’ll be limited to the new OS.