well, last week i took a class and invited 4 of the students to gmail... so maybe that had some bearing on the invite increase, seeing that i was "spreading" gmail as they wanted... i went from 4 to 2 last wednesday, back to 4 on thursday, then down to 3 on friday (but i found a bug, since i entered any actual email thread and it showed "invite 4" but only displayed "invite 3" when i looked at the thread listing).
but besides that, they did change their layout and all of the sudden i went from 4 invites once again to 50. all i can say is "wow"