Microsoft began sending Windows 7 SP1 test invitations.

Microsoft today began sending Windows 7 SP1 beta test invitations to testers. The Windows Customer Connection Team is selecting excellent testers to experience the latest version of the patch, the first release. Available in English, German, Japanese, French, and Spanish, Windows 7 SP1 also works on Windows Server 2008 R2. It includes all the hotfix patches and will also bring RemoteFX features to the server system to enhance the remote control experience.

Before there were early versions leaking on the Internet, SP1 is also rumored to support the latest versions of USB 3.0 and Bluetooth, Wi-Fi stacks.

There is news that Microsoft will announce a public version of SP1 at the end of July, so I believe that the scale and timeline of this round of testing should not be very large.