I usually top out at around 27 - 28 MPH, but that's because the steepest hill on my commute has a stoplight at the bottom of it, so it's best to keep my speed in check. And anything over 30 on my fixed-gear is not really a comfortable spin. I've exceeded 30 a couple times down that hill, but normally keep it in the high 20s.

I took a slightly different route a few months ago when I was house-sitting for a friend. That route took me down a longer, steeper hill with a nice long run-out at the bottom. I hit 40MPH on my fixie (geared 42x16) down that hill, but was spinning like mad and barely in control. That scared the **** outta me, so subsequent trips down that hill involved judicious use of the front brake and topped out at around 35. Still a pretty fast spin...

-Trevor