The Windows 7 Team blog reports that there are up to 5 test updates being rolled out to Windows 7 beta testers, these updates will be delivered using the usual Windows Update method but contain no new features or fixes. The Microsoft Update blog explains this a little more:
Starting on February 24th we will be offering some test updates to Windows 7 Beta (build 7000) computers. We do typically verify update scenarios during a Beta, and releasing these test updates will help ensure that when we need to release real updates, the process will run smoothly. There will be at most five updates. These updates test the Windows 7 servicing infrastructure. They will not deliver any new features or fixes.
The updates will be offered interactively. This means that users will be notified of available updates, but they won’t install automatically. Users will need to go to the Windows Update control panel and manually start installation. They will also be clearly titled as a test update. If you don’t want to install the updates then you can right click on each update and select “Hide update” to prevent it from being shown in the list of available updates.