From the Vembu blog:

Create a wave of your own and make it public by adding public@a.gwave.com as wave participant. There’s a small ‘trick’ needed with this one. When you Click on the + beside the Manage Contacts link and type public.a.gwave.com as a contact, you’ll get an unfriendly  “User does not have a Google Wave account” message. Just ignore it, and hit the enter key. The contact gets added and is called ‘Public’ in your contacts list. Now you can create a new wave and add ‘Public’ to the conversation.

Bloody intuitive.

I don't think anything like ignore it and hit Enter would've occurred to me on my own.

Small hand motions, small, small... that's how we learn to Wave!

Cheers...