The guys at TechCrunch have been thinking the same thing we have been over here: With all these invitation-based Web 2.0 betas flying around, someone could do the world a service by building an exchange for them.
Hence, InviteShare, a site that does just that. If you want an invite to a service (such as the current darling of the digerati, Pownce), you can put your name on the list and someone with passes to spare might help you out.
Many sites, once you're invited, also give you invitations you yourself can give out. Thus InviteShare can keep growing.
If a site is gating access via an invitation system, though, InviteShare could unravel the growth plan and potentially bury a fledgling site in more users than it is ready for. However, there is nothing to guarantee that a site that you are invited to will give you invites that you can put back in the system. Many early invitees to Pownce found they couldn't invite friends for a few days, until Pownce released a new batch of invitations into the system.
Ironically, this site was slow and unreliable this morning, buried under new traffic. If only there was a way its founders could limit traffic to the site while they debugged their code...