The fight might not have a date or location yet, but oddsmakers aren't waiting for trivial details to get the betting started on a UFC welterweight title fight between Georges St. Pierre and Johny Hendricks. And it seems that the books believe Hendricks is the toughest challenger GSP has had it years, at least by the look of the first set of odds. Here they are (via BestFightOdds):
The odds haven't been this close for a Georges St. Pierre fight since he defended his title against B.J. Penn back at UFC 94. The GSP/Thiago Alves line for UFC 100 could be found in this range as well if you really looked around, but Penn/GSP was closer betting-wise overall. If you want a more contemporary comparison, a few books offer odds on the mythical Anderson Silva vs. Georges St. Pierre fight, and the odds are about the same as this bout in reverse (GSP is is the +180 to +200 underdog).
Both GSP and Hendricks won their bouts at UFC 158 and their matchup is all but confirmed, though the when and where are up in the air due to the champ's injury concerns. As far as betting on this, I wouldn't touch it right now. Only one book is offering it (Sportsbook) and other sites might not open in the same range. If you're looking to bet on Hendricks, you'll probably get a much better fight closer to the actual bout.