Xcode 4 GM
gcc version 4.2.1 (Apple Inc. build 5666) (dot 3)Apple clang version 2.0 (tags/Apple/clang-134) (based on LLVM 2.9svn)
You can use a later gcc version in Xcode 3. I did it once, however I don't fully remember how; I used a gcc I built myself from the svn trunk. There is post here which might help you.
You can also build and run later versions of clang within Xcode. See for example:
http://shapeof.com/archives/2010/01/using_the_latest_llvm_with_xcode.html