This is from Technet:
Guidelines on usage:
Protect your PC and data. Be sure to back up your data and please don’t test Windows 7 on your primary home or business PC.
You have 10 days to activate the product. If not activated within 10 days, the system will shut down once every hour until activated. Unsure on how to activate? Visit our FAQ.
The 90-day Trial is the full working version of the Windows 7 Enterprise, the version most of you will be working with in your corporate environment. It will not require a product key (it is embedded with the download).
The 90-day Trial will shut down once every hour when you have reached the end of the 90-day evaluation period.
The 90-day Trial is offered for a limited time and in limited quantity. The download will be available through March 31, 2010, while supplies last.
After the 90-day Trial expires, if you wish to continue to use Windows 7 Enterprise, please note that you will be required to purchase and perform a clean installation of Windows 7, including drivers and applications. Please keep this in mind; Windows 7 Enterprise is not available through retail channels.
Technical details/updates/questions: Please review our FAQ or visit the Windows 7 support forum.
Stay informed. You can keep up with general technical information and news by following the Springboard Series blog. Want technical guidance, tips, and tools? Visit the Springboard Series on TechNet.
Keep your PC updated: Be sure to turn on automatic updates in Windows Update in case we publish updates for the 90-day Trial.
Source: Windows 7 Enterprise 90-day Trial
I have been using the RC copies, and am fairly certain that I was informed that I would be able to purchase the key at a later date if I wished. I know Microsoft will have your information – but perhaps this might help speed your search a bit. Good luck!
Windows 7 Release Candidate
Notice: Downloads will end August 20
Thanks for all your help building Windows 7. We are on our way to the last big milestone: Release to Manufacturing. Starting on October 22, you’ll see it on PCs and store shelves. As we gear up for that, it’s time to wind down the Release Candidate testing program.
If you or a friend needs a download, you’ve got time. We’ll continue to offer the RC download until August 20, 2009. After that, you won’t be able to get the download, but if you have the bits, you can still install the RC and get a key if you need one. (To get a key, just go to the Downloads page and follow the instructions.)
Got the RC and need help? Experts on the Windows 7 Forum can help answer your questions. The forum also gives you a way to share what you’ve learned with other people.