Is it a coincidence that GMail and Google Wave where invite only also for a long time. They got a lot of publicity when people where trying to chase the invites from different sources. It's always so that when you can't get something you'll chase for, it becomes more interesting. Could this be the strategy with Google+ also?
Also when you eventually get Google+ account, you can basically invite anyone to use it, without any limitations. So if they would be scared of capacity problems, maybe they would have limited invite possibilities in some way.
Still there's a second change. It maybe that Google guys have noticed that invite only is the best way to limit the speed of adaption of the service and that way control their capacity better. Pier to pier inviting is more predictable than plain open service adaption.
Whatever the reason is, still Google+ invite only is a perfect marketing way. It will raise awareness of service when people who are interested are chasing for invite or when they have received an invite.
Written by +Henri Hämäläinen