This information might be useful for those who are not members of the $99/year iOS Developer Program. I am not a member, I have Snow Leopard, and I purchased Xcode 4.0 for $5 from the Mac App Store in March 2011. Just for fun I searched the app store for Xcode. As expected, the only search result is Xcode 4.2 with stated minimum requirement 10.7 (Lion). I am curious why it even shows Snow Leopard users the active Free button that implies I can download it. I don't know how the app store works but I assume it would not let me install this on 10.6 Snow Leopard.
But the Purchased view shows Xcode for Snow Leopard. I noticed the button on the right showed Update rather than Installed. Clicking the item name to find details including the version number shows Your request could not be completed. I decided to update in hopes this would be 4.2. I deleted Install Xcode from my Applications folder, restarted the App Store, then pressed the Update button. 1.77GB later I have Install Xcode in my Applications folder, preview shows version 4.2 (used to show 4.0.2),
The installer just finished running, I now have Xcode 4.2 build 4C199. About Xcode.pdf in the Developer folder starts with Developer tools for Mac OS X 10.6 Snow Leopard and iOS 5. So this is clearly not exactly the same as Xcode 4.2 for Lion. It does include the iOS 5 SDK, LLVM 3.0, etc. Not sure what is different in the Lion build.