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 10.6.6 -- it's already been established that Snow Leopard is required for the Mac App Store, and the small "dot" upgrade to 10.6.6 likely contains the OS-level hooks necessary for compatibility with the new store's one-click install, copy protection and updating features.
Of course, that also means we might have to wait until 10.6.7 for any fixes to the things that the 10.6.5 and 10.6.6 updates may break on some people's systems...
Apple Mac OS X Snow Leopard