The listing, INSTANT Google Wave Invite has ended.
comes with a direct link to sign up!!!
"Thank you for signing up to give us early feedback on Google Wave. We're happy to give you access to Google Wave and are enlisting your help to improve the product.
To accept your invitation, sign into Google Wave at the following link*:
https://wave.google.com/wave/inviteXXXXXXXXXXX
(If you do not have a Google account, you will be prompted to create one)
Once you've signed in:
Check out the videos and example waves
Invite others to join you
Happy waving!
The Google Wave Team"