Apple has just posted another GM seed (number 2 if you're keeping count) of Xcode 4. Build 4A304a is ready for you to download, install, use and love. You need to be a paid member of either the iOS or the Mac dev programs to access this build.
Now, if Apple would only get around to, you know, releasing Xcode 4..., it would make life ever so much more interesting.
Be aware that if you are beta testing Lion, you will need to download and install the separate Lion-specific tools. Xcode 4 GM is built for Snow Leopard.
A tip of the developer's cowboy hat to Morgan for telling us about the re-release