According to Apple’s release notes, the update addresses a problem that causes photos to be moved to a new event called “From MobileMe” during the migration of albums from a user’s MobileMe Gallery.
The new iPhoto upgrade also fixes a (rare, says Apple) problem in which iPhoto can hang when upgrading your photo library.
The update is available now via Software Update, and weighed in at more than 600MB on two different Macs in our office.
The update is compatible with OS X Lion 10.7.4 or later.