It was weird. Yesterday I read all these stories saying the beta was already full and this morning I had an invite waiting for me in gmail. So I'd say there is still a chance you'll get in if you signed up. They may be slowly sending them out, rather than sending them all out on day one.
By the way, it's really cool, but kind of weird editing a wave for my own enjoyment. There are some interface issues, but the way it works is pretty cool. A wave could quickly turn into a mess if people aren't careful about where they put their replies.
For instance if I typed:
"I will punch your face"
Someone could reply to the word "punch" and you get something that looked like this:
Scott: I will punch
BadReplier: I don't want to be punched.
Scott: your face.
The original message is pretty difficult to read. You could use the "playback" feature to see the original message, but that could be annoying depending how many times the wave was edited.