We have a long list of people who have either signed up for an invitation to Google Wave, or who have been nominated by a friend to get a Google Wave account. Unfortunately, we aren't able to pass out invites as fast as the nomination list has grown, so we're not currently adding more people to our waiting list.
As soon as we're confident that the system can accept more users, we will add a wave to your account that allows you to nominate friends and colleagues for an account. Once you have the capability to invite people, the wave appear in your inbox.
Just got my invite to Wave this morning. Kinda weird but it says I only get 8 invitations and like JF mentioned it also says "Invitations will not be sent immediately. We have a lot of stamps to lick". I will trade anyone a Wave invite for a Voice which I never got but be warned that it is not instant and may need some sort of approval before they send one out. Anyone interested please PM me. Thanks Jon