I think requiring a cell phone code to sign up for gmail is holding a lot of people back. Thos who are comfortable using cell phones and text messages probably wouldn't mind too much but I don't feel many people would go out of their way to use a friend's cell phone in another situation. As for invitations from another user-- I'm not sure if this is the case for eveybody, but I haven't had my invitations restocked for quite some time, and I used the last of them a long while ago. Has anybody had their invites restocked recently?

Google claims to require a cell phone invite to keep out robots which makes sense, but is there another way?