With Windows 7 going to be released this year it seems that Microsoft has already starting working on the next version of Windows,Windows 8.

Here is the extract from the Microsoft Job posting site

Are you a talented SDE/T Lead looking for the next technical challenge on a key operating system component? Are you passionate about distributed systems, networking, file systems and want to be the leader of a cutting edge project? Are you looking for a team with abundant opportunities to grow? If so, we have the position for you!
DFSR is Microsoft’s premier file replication engine and is an integral part of our branch office strategy and File Server role. It can scale to thousands of servers and replicate hundreds of terabytes of data. We have shipped the technology that powers file sharing in Windows Live Messenger, Windows Meeting Spaces (Vista) and Branch Office replication in Windows Server 2008 which has strong customer deployment. DFSR technology saves MS-IT and our customers more than 80% WAN bandwidth by using advanced On-The-Wire differential compression.
For the upcoming version of Windows, new critical features are being worked on including cluster support and support for one way replication. The core engine is also being reworked to provide dramatic performance improvements. We will also soon be starting major improvements for Windows 8 where we will be including innovative features which will revolutionize file access in branch offices.

The last paragraph clearly states that they have started working on Windows 8.

We would be posting soon as and when more information is available