*Post Updates At Bottom
The obvious reason you haven’t received your Gwave invite yet, is the fact that nobody has invited you 
But if you’ve managed to wrangle yourself an invitation from one of the 100,000 preview invitees or the original dev sandbox invitees, originally handed out by Google, here are a few thoughts on why you haven’t got in yet.
All that I’ve talked to on Google Wave were awarded 8 invitations to give out. Some have claimed that certain big wigs got more, but I haven’t personally talked to anybody who has.
Once you invite somebody into Gwave, it says that they have been ‘nominated’ which to me means that the invites are placed into a queue somewhere. Some said that a few of their ‘nominations’ have got in but I’m not sure that these people weren’t already among the original Google invitees and it just took them longer to accept their invitations.
If you read the image above it says “Invitations will not be sent immediately. We have a lot of stamps to lick.” What I think Google is doing is using new Google Wave users gradually in order to test how robust the system is and to fix errors along the way. They have a big influx of new users and I’m sure they are experiencing bugs that they hadn’t foreseen. Note it also says above “Keep in mind that this is a preview so it could be a bit rocky at times.”
I believe that, as they shore the system up, they will begin accepting the new nominations in increments that they believe they can handle without crashing. I’m imagining the ‘nominations’ will receive their invites in a week or so, but this is just my guess.
Personally, I wish I had 100,000 invites myself to give out.
I can’t wait to see you on the inside.
*UPDATED Oct, 3rd
My original thoughts have been somewhat verified by a few Google employees working on Wave. First of all special thanks to Anthony Baxter, senior Google software engineer, for commenting on this post and letting us know that I was on the right track.
There were also these two tweets from Lars Rasmussen, who led the development of Google Wave, and lead Google Wave project manager Stephanie Hannon, which stated things along the same lines.
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I’ve also verified from inside Google Wave that some of the ‘nominations’ have indeed received their invites. Will Moody was nice enough to let me quote his Google Wave email invitation.
XXXX XXXXXXXXX has invited you to preview Google Wave!Google Wave is a new online tool for real-time communication and collaboration. A wave can be both a conversation and a document where people can discuss and work together using text, photos, videos, maps, and more. Learn more at wave.google.com. This is still an early preview of Google Wave, so you may run into some bumps along the way but we look forward to your feedback.