Im not exactly sure how this happened so quick unless im missing something? Anyways i only just recently moved my largest community from vB4 with a PR2 on the forum index page (forums/index.php) maybe a month ago.. Last night when i went to bed google hadn't even noticed my new XF site with a new URL (community/index.php) I didnt bother with any redirects to any of my forum pages let alone the forum index page.. Anyways this morning i woke up to a nice little surprise with a google PR3 on my new XF forum index page.
Intersting mine was bumping back and forth between a 2 and 4 every other month.. I just figured it would take the better part of a year to get any PR from Google with the new URL on the forum index not using any redirects. I really was expecting the worse case scenario playing out. I suppose im going to have to re read up on the factors involved in determining PR.