Take a peek at Flight Aware for almost any Class B at which you look. Right now, KORD, 4000 msl 280 kts for a SW 737. Night, junky weather, 1am local, so not much glider worries. But, it has been my observation there are not many air Traffic Cops worried about Part 121 speeders. Perhaps that is an entirely different thread.

Cindy B, excellent points made in your post.
Regarding the speeds, I have found ATC very diligent at monitoring speeds. I've been asked to verify my speed of 250 Kias or less, several times when I am right at the limit. ATC does not even have authority to approve more than 250 Kias. They can approve an increase above the 200 kias limit under class B or in the vfr corridor of class B, but the 250 kias limit is a hard limit. Looking at the flightaware speeds is a little misleading because those are ground speeds that are effected by wind speed and direction. The speeds are also computed based on position change, so it is an approximation of ground speed. There is no way to know exactly what the indicated air speed is for the aircraft at a specific point, unless you are in the aircraft. Flightaware is great to see the patterns of flights in a general area, see the typical altitudes, and to realize how fast some of the aircraft are traveling at low altitude.

Again, because many of us throw the 250 number around so frequently, most pilots forget that the number is "indicated airspeed"; and that 250 kias at 10,000 ft, is really 300 knots.

For those that wonder why the controllers want everyone low at the same alititude, remember the systems they are using. It is a good idea to go visit a center and/or tower. The systems are all a flat screens that are great at projecting potential flight paths at the current speeds. The systems and the humans are great at visualizing these horizontal components. It is more difficult to visualize and manage the vertical component of altitude. Likewise, aircraft can maintain an airspeed very well which makes horizontal speeds more consistent. Vertical speeds are more difficult to predict and control by the computers and controllers. Every center and tower I have visited have been staffed by some of the nicest and most professional staff you will ever meet. I have witnessed a few stressed and angry controllers on the radio, but the vast majority are a great resource and well trained. It would be a shame to see it privatized and screwed up.