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 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-releaseApple releases Xcode 4 GM. Again
