Apple apparently posted the gold master of Xcode 4 this morning -- that's the final pre-release version before the final version is eventually offered up to the public. Unfortunately, just as soon as the new version of Xcode went up, it was pulled right back down again. Reports from developers who saw the release's Read Me say that it apparently listed Mac OS X 10.7 Lion as a requirement to run, and as you probably already know, that version of the OS isn't actually available yet. So one of two things is happening here: either there was a typo in the documentation, and Apple just meant that Xcode 4 requires 10.6, or Xcode 4 really does require a version of Lion, and a beta of that operating system will be available to developers soon.
At any rate, we do know Xcode 4 is officially on the way. Apple's Developer Tools evangelist says on Twitter that we can expect it out very soon. The new version will bring a single-window UI for developers of Mac and iOS apps and a bunch of other new features for them to play with. Stay tuned, devs -- there'll be new goodies on Apple's dev portal soon enough.