From May 18 2010 Google wave is available for all. You need not to have a wave invitation to create Google wave account. Just go to wave.google.com
If you tried Google Wave out a while ago, and found it not quite ready for real use, now is a good time to come back for a second try. Wave is much faster and much more stable than when we began the preview, and we have worked hard to make Wave easier to use. For example, you can now get email notifications when waves change, easily navigate to unread parts of a wave, and remove participants added by mistake. We have also added permission management options and an extensions gallery.
Read more from the official Google wave blog