Correct. By default, you have a 30 days trial before having to activate Windows 7, or Windows 7 goes into a reduced functionality mode on the 31st day.
If you have a free Windows Live ID or Hotmail account, then you can get a Windows 7 RC product key number for free directly from Microsoft at the link below. After you select the language and version of Windows 7 to download and fill out the form, you will get a product key number. Windows 7 Release Candidate Customer Preview Program