updated 09:16 am EDT, Tue October 8, 2013
Syrah currently almost identical to Mavericks GM
Apple is already seeding nightly internal builds of OS X 10.9.1 and 10.10, according to sources and leaked data. 10.10 is said to be codenamed "Syrah," after a type of wine, and have begun seeding in small groups in early September. Seeding has scaled up in recent weeks though, and web traffic from 10.10 machines has reportedly escalated in recent days.
Although Syrah is ultimately expected to get a redesign to copy the look of iOS 7, current builds are described as almost identical to the GM build of Mavericks. Work on the OS is so early that no user-facing features or interface changes are thought to have been implemented. About 30 seeds are said to have been issued so far; for comparison, by the time the first Mavericks Developer Preview was released, Mavericks had already reached build 476.
Following the debut of the Mavericks GM Apple is said to have moved a large portion of its software engineers over to Syrah. Some people though are said to be at work on the first update to Mavericks, v10.9.1. That release is expected to concentrate on bugfixes, but might also add the ability to block iMessages and FaceTime calls from specific users.