As widely expected Apple has started seeding Mac OS X 10.5.1 to developers for testing. The seed carries the build number 9B13.
Apple states that the following areas should be targeted for testing; Mail Sync, Spotlight Index, Disk Management, Text Drawing, iCal and CalDAV syncing, Keychain login, Read-Only Issue with SMB, AirPort 802.1X, Application Firewall, To-Do Notes, and Smart Mailboxes.
[Update] It appears that AppleInsider is able to shed more light on the seed.
The rumour site reports that the beta software weighs in at at a "hefty" 267MB.
Also reported is that Mac OS X 10.5.1 delivers a "significant number" of bug fixes and over two dozen fixes and code corrections.
Apple has reportedly asked that its developers test the update broadly, paying particular attention to Time Machine, Mail, iCal, Back To My Mac, Bonjour, AirPort, gaming graphics, networking and the Finder.
The timeframe for a public release of Mac OS X 10.5.1 is reportedly a few weeks.