Can't upgrade to 10.7.2 over a previous iCloud beta seed
While i haven't seen this problem with Lion, it was common with previous versions of OS X. The way to fix it was to download the Combo updater from Apple, and run the update. Apple provides this for Lion at http://support.apple.com/kb/DL1459, so i'd download that first, run it, and see if it fixes your problem.