Same here, we're having significant traffic increase on phpBB SEO since last Google dance, despite the fact the home page dropped from pr5 to 4.
Looks like we where penalized a bit in PR, but I'm not sure about the real meaning of this penalization.
In last September, we had over 3000 Google backlinks, with about 1 000 000 yahoo backlinks, then, after the October Google dance, we dropped to 2640 Google backlinks despite the fact we went over 1 500 000 backlinks on yahoo by this time. The Google webmaster tool as well shown by them a significant increase in backlinks by then.
And now, we've all the sudden reached 13700 Google backlinks, but we only reached 2 000 000 yahoo backlinks by then.
The odd thing is that Google backlinks decreased in October.
So, for Google, we were having less backlinks upon last gd, and then suddenly about 10 000 more in January, which could explain the penalty, but the strange thing is : why did we drop in the first place during October's GD ?
The logic would have been to follow a more regular growth than that IMHO since we did not change anything to our backlinks and they were increasing on a regular basis.
So some Google bug could be involved in the October GD for phpBB SEO, this would explain the PR penalty.
Anyway, it's not very important since we blown up all traffic record in january so ...