Once apon a time they were hard to get, now there are more than 280,000 available at ISnoop.net at their Gmail invite spooler. I gave away a few a long time ago when they were still semi-scarce. It's cool to see that they have given away more than 300,000 so far.
Update, 24 March
They now have 591,460 invites available, with 605,389 shared so far. That's like double a month ago. Very impressive.