I agree with others that say no speed limit below 10,000 feet in Houston. I've flown out of IAH to DFW and the departure controller will say "no speed limit". That means he wants us to go faster than 250 kts in the climb. That means in the RJ you can climb at the 335 KIAS Vmo (oops, did I say that, I meant 290 KIAS which is SOP). I was with one captain who responded to the departure controller with just a "Roger" and he only wanted to climb at 250 Kts. A couple minutes later the controller came back on and said, "SkyWest, say airspeed?" When he found out we were still at 250 kts he was mad and said if we still wanted to go 250 kts below 10,000 feet then we had to tell him, otherwise as soon as he authorized it then it was expected. They use the higher speed for getting departures out quicker and they plan spacing accordingly. I also agree about the class B speed limit. There is no speed limit in class B, just under 10,000 feet. Also 200 kts below Class B as well as within 4 nm and below 2500 feet of the surface of a Class C or D airport.