Those brave souls who ignore the "reminders" and work their way around the shutdowns will face one more phase in the expiration process. On June 1, if you are still on the Windows 7 RC, your license will expire and the non-genuine experience will be triggered. This means Windows will boot to a black desktop and users will still have to deal with the constant notifications and bi-hourly shutdowns.
As annoying as all this may sound, especially for those who have been running the test release as their main installation, we can't say Microsoft wasn't clear about the whole expiration process from the get go.
That said, if you need some more time to back up your stuff there is a little-known (and perfectly legal) way to extend your RC license as many as 90 more days -- just open a command prompt window with administrator privileges, enter "slmgr -rearm" (minus the quotes) and reboot. The 'trick' can be used up to three times and each will give you 30 days to go grab a legal copy of Windows 7 and upgrade.