What people are forgetting is that this system is a closed proprietary system, unlike GMail. Yes, invites to GMail were scarce, and if I remember correctly even sold for hundreds of dollars on ebay. The difference between GMail and Google Wave (GWave? yes/no? ;) is that when you were invited to gmail, you were invited to use a new "interface" of an existing and mature protocol - email. You didn't have to wait to "show off" your gmail account to your friends, you simply had to send them an email.
Google Wave requires mass adoption of something that is inherently closed (unless they open this protocol to other companies) and so will suffer from a slower adoption rate, and as a consequence, a lower utility rate, even if the invite system ceases to exist at all.
To be honest, I'm not sure if this is an issue. Many of my friends who got invites seem to think that Google Wave is going to revolutionize _chat_, but I don't think this was ever the claim. It may yet revolutionize collaboration, but in order to collaborate effectively you don't need to just sit there and wait for people to log on -- start a wave, invite some friends, and watch it evolve.
edit: to follow up on my 'non issue' comment. My point is that for Google Wave to be useful, only those who wish to collaborate together will really need to adopt it, not your entire google contact list. If a company signs up their domain for google wave, you can be almost certain that they will make much better use of it than I will with my group of friends on an individual level