The Mac OS X 10.6.5 update hasn't been publicly released yet (although it's expected any day now), but according to 9to5 Mac Apple is already seeding builds of 10.6.6 to developers.
This is indicative of a few things. First, OS X 10.6.5 is likely finalized and ready for public release. Second, the tiny size of the 10.6.6 update (only 3.7 MB) and its small list of changes (mostly related to the forthcoming Mac App Store) indicate that the 10.6.6 update will likely come relatively soon after 10.6.5's release.
Finally, it's probable that access to the Mac App Store will require updating to OS X 10.6.
