You can now hide apps from your Mac App Store Purchased list. These buttons do not delete apps from your Mac, they just help keep your Purchased list tidy. I suspect the buttons are a new addition in 10.7.2, but I could be wrong. Anyone on 10.7.1 or earlier see them?
If you want to restore apps you’ve hidden, go to your account area, click “View Hidden Purchases,” and click the “Unhide” button.
Update: One commenter says these appear in 10.6.8, so maybe this was just a coincidental update. After all, like iTunes Store, the Mac App Store is mostly a webpage wrapped in an app, so Apple can make some changes like this without requiring a software update.