As Nick posted on the Windows Vista blog last month, the Vista beta and RC preview versions will expire on May 31, 2007.
"On 31 May 2007, all Customer Preview Program (CPP) pre-release versions of Windows Vista will expire. If you are running a pre-release version of Windows Vista (Beta 2, RC1 or RC2), you will begin to receive warning notifications about the upcoming expiration on 18 May. To avoid work disruption and loss of data, we strongly recommend that customers running any of these pre-release versions of Windows Vista migrate their PCs to the final version of Windows Vista prior to 31 May.
"For more information on migrating away from CPP versions, please go to http://www.windowsvista.com/preview.mspx. We've included below a listing of frequently asked questions to help those of you still running a pre-release version of Windows Vista on your PCs to get started."
For more info, please see Nick's post - there is also a comprehensive FAQ included.
Added May 13: Also, keep in mind that if you have one of these trial versions of Vista installed, you'll have until August 28 to back up any important data from your PC.