I really think it depends on whether you need to change terminals or not. I had about a 45-50 change time and had to go from C to B. We headed straight for the new gate with only a stop at the bathroom. We were there maybe 15 minutes and then they started to board. Now, if end up in the same terminal, you will be fine, granted if the flight is on time.
Our flight was on a small 40 seat Continental jet and our second flight was on a 757.
30 minutes maybe tight. Continental now uses several terminals at IAH and it gets somewhat hectic at times. I'd err on the side on caution an give myself a minmum of 45-50 minutes but preferably 1 hour.