You might have heard that the Windows 7 Release Candidate would be usable through June 2010—quite the nice free trial. But it starts asking for an upgrade, and automatically rebooting, in March 2010.
That's from a Microsoft spokesperson, who specifically said that "bi-hourly shutdowns will begin March 1, 2010." Those shutdowns come without any prompt, unlike the standard restart requests accompany system upgrades. If you're still using the RC after the drop-dead date of June 1?
On June 1, 2010, if you are still on the Windows 7 RC, your licence for the Windows 7 RC will expire and the non-genuine experience is triggered.
For those lucky enough not to know that that means, it's a black background, and almost constant reminders that you're running a copy of Windows that isn't Genuine.
If you're still using the Windows 7 Beta that came out in early January, you'll get the same kind of auto-reboots and upgrade notices on July 1. All in all, don't plan on using a beta or release candidate of Windows 7 on any system you're doing actual work on. Windows 7: customise How the Date is Displayed in the System Tray [Tech-Recipes]