The paradox [or maybe just conflict] of the closed alpha.
On one hand Google loses a lot of buzz and momentum every day they dont open things up. As you said, early invitees are getting bored and the tech conversations only last so long.
On the other side you've got the risk of a site that isn't ready for primetime and scares of the general populous. Since there is clearly an alternative out there it's really a one shot, win or go home proposition.
While I don't have the benefit [or curse as the case may be] of all the feedback & usage data, my $0.02 is that the gates should be opened up while there's enough people pushing to get something going.... Usually the anticipation of slow acceptance works to build up traction but this isn't a small venture and the only way to get a user to stick is to be so much bolder and move entire social circles moving over in very short order.
Reasons not to do this would primarily be load [no uptime, no bueno], impending game changers [the feature that makes everything so far look unimportant] or the realization that the system as is is not a game changer [why blow your shot if you know you missed it].
Like any network, Plus fails if people are still checking the old site and preferring what they see so momentum is critical. Not to mention it gives Facebook even more time to launch and push features that rival, or supersede anything plus does.