Brandon over at the Windows Team Blog has posted a notice about upcoming test updates that all Windows 7 Beta users will see starting February 24th. The updates will not provide any new functionality or fixes. They are designed to test the update infrastructure in Windows 7.
Starting February 24, we will be releasing up to 5 test updates to PCs running the Windows 7 Beta (Build 7000) via Windows Update. These updates allow us to test and verify our ability to deliver and manage the updating of Windows 7. We typically verify servicing scenarios during a beta.
Read More on the Windows Team Blog