updated 10:30 am EDT, Wed July 20, 2011
Likely to be replaced with 4.1
Although OS X Lion is now available on the Mac App Store, Xcode 4 has temporarily gone missing, browsing shows. As of this writing the software isn't even discoverable through search. The suite is the primary tool for development of OS X and iOS apps.
Apple may have pulled the program while it prepares to upload Xcode 4.1, which recently went gold. Among other enhancements, the update should be optimized for Lion. Apple is already seeding a third preview of Xcode 4.2 to developers.