Given the fact that the turn-around differs every year, I don't really think there is any such thing as a course record (except perhaps for the women <g>).
EDIT: Fastest time for any man in the past decade appears to be the 52 flat that Joe Hill did in 2004. However, according to my SRM the course was only 38.56 km that year, vs. the 41.27 km we raced last year. That would make Henderson's time last year the fastest, as it would be the equivalent of a 51:36...but as you know, he apparently went through some sort of space-time warp on the return leg (maybe he got a big boost from that storm?).
EDIT2: There is also Andrew Chocha's 52:26 in 2007. I don't have an exact distance handy (as Angie's speedo quit working), but it would have been very, very close to 40 km (based on her time, cadence, and the roll-out of her fixed gear).