Apple's annual developer conference in San Francisco.
Apple Continues Mac OS X 10.5.3 (9d27) Development
Apple appears to have begun seeding these test versions more frequently than in the past. Historically, Apple has waited until development was wrapping up before accelerating their seeds.
Regardless, we've heard anecdotal reports that even the latest version has stability issues which means Apple still has some work to do. Once released, the 10.5.3 update will be available for free through Mac OS X 10.5's Software Update.