Apple has seeded developers with a new version of Mac OS X 10.5.5 (Build 9F9) to test. The latest seed comes just one week after the prior seed.
The newest seed lists 24 new fixes addressed in this seed with a focus on Sync Services, Mail and Addressbook.
Apple has started aggressively seeding Mac OS X builds shortly after the last release. The last version of Mac OS X was released on June 30th, 2008 and provided support for MobileMe services.