According to Dave Meltzer, the January BJ Penn vs. Georges St. Pierre fight will be for the welterweight title:  

St. Pierre vs. Penn will be for the welterweight title, giving Penn the chance to become the first person in UFC history to hold championships in two weight divisions at the same time.

This is great news from my perspective, I really hated the idea of another three round fight between these two.  It's hard to imagine this going to decision now, so we should get a real answer as to who is the better fighter at 170 pounds.  This also has to favor St. Pierre, who is in far better condition than Penn, and pushes a tougher pace than any other fighter in the UFC.