I must ride in an exceptionally hilly area. I routinely find myself north of 40mph on all of my bikes.

Fastest ever 53mph - descending Golden Gate Canyon between Golden and Boulder on my Cannondale SR500 - Try as I might, I haven't been able to top this one.
Fastest during commute 51mph - descending Crowfoot Valley hwy between Castle Rock and Parker on a fully laden Gary Fisher Kaitai - Stable as a rock and verified with GPS and cyclometer, though I think I also had a stout tailwind that morning.
Fastest with current stable 51mph - descending South Lake Gulch road south of Castle Rock on the Raleigh Team - Had to back off a when I started to feel a little woble in the front end.

Once I'm spun out, I get into a good tuck with my weight shifted to the front as much as is comfortable and let gravity do the rest.