I installed vBSEO site map generator about 4 months ago and it actually drooped my Google PR from 3 down to 1. After a talk with the hacks designer I was told that it is normal for the PR to go away for a few months until the bots re-index all the new links.
I'm still waiting for my PR to come back. The good thing is that now alot of my threads are indexed in Google and Yahoo. My traffic has also increased by over 400% since I installed the site map generator.