I, like many, was having the issue in the Mac App Store with Lion saying that it was already installed - even though it was not. I tried my fair share of option-clicking my way through the App Store with no luck. I had a Lion Developer Preview 4 install file on a storage hard drive hooked up to my Mac. As soon as I deleted that install file from the storage hard drive, I was able to successfully purchase and download Mac OS X Lion. Just another thing to try to anyone having this issue.