I don't know if this might help you but I somehow noticed this bug as well:
Whenever I get new updates, I do the usual thing: Tap on Marketplace, then tap on the "# updates" notice, then "update all" but here's the key factor; instead of tapping the home button, just go back home by tapping the back button till you get there and the update number will be gone, if you don't do this and just tap on the Windows logo, the number of update will remain there on the live tile (or at least that's what ALWAYS happens to me)
10-19-2011 03:20 PM