Thanks for using Gmail and helping us improve the service. We're ready to expand our test to a few more users, and because you've been a trusted early tester of Gmail, we're looking for your help. Please invite a few more people who you think would like Gmail and could help us make it even better.
They've given me two invitations to use, comment if you want in.
UPDATE: These are long gone, email asking to beinvited will be ignored.