Houston had a test program in place allowing aircraft to exceed 250 knots below 10,000 feet on departures. That program ended, and now IAH departures must maintain 250 below 10k, just like every other airport.

At 10,000 feet we are allowed to accelerate to our optimum climb speed, which for most aircraft is near 300 knots. If you are restricted to an altitude below 10k for awhile (descending traffic above you), or speed restricted due to an aircraft right in front of you, you will be kept at 250 knots.