I carry my Colt OM .45 IWB at 4:30 and never have any problems. It the summer I just ware a t-shirt, or an outside short sleeve shirt, and it is well concealed. But as, Excaliber, said the most important thing is to get a high quality holster and use a high quality gun belt, at least, 1 1/2" wide for stability and you should not have any problems. I had to learn that last part the hard way, after several cheap belts and holsters.... for comfort and concealability a quality holster and belt is the only way to go.
-geo