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?!

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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.