Deadline for those still running the beta version, as opposed to the release candidate. Starting July 1, that software will start shutting down every other hour.
The release candidate also won't last forever. It is set to expire on June 1, 2010, with the bi-hourly shutdowns starting March 1. And, Microsoft also says that those moving from the release candidate to the final version of Windows 7 will have to do a clean installation! Be prepared.
