You can now invite any email address to a wave.
To invite someone who doesn't have a Wave account to your wave, just click on the + sign in the participants bar and enter their email address.
That's it! They will get an email inviting them to join your wave and you will see a dummy profile your wave. This profile will be replaced with the user's profile once they log in and view your wave.
Note: This will only work when you add contacts through the + sign in the participants bar in the wave. The Contacts panel (which is usually on the bottom left corner of your Wave window) will only accept @googlewave.com alias or accounts that already have Google Wave.