Okay I figured it out--at least my specific problem. The cause of the slowdown and distortions was non-power-of-2 textures sizes. While Qt on Linux and, I think, previous Qt versions on the Mac had no problem with textures of arbitrary height/widths, with Qt 4.6. this was the difference. When I rewrote my code to force power-of-2 sizes, the problems disappeared. The only caveat is that I was not able to repair the 2D painting example by changing the OpenGL image from 200x200 to 256x256 pixels. The image got larger but retained the same distortions. Lastly, according to the 2.1 red book, only OpenGL versions prior to 2.0 have a problem with non-^2 textures. Has Qt 4.6 regressed to an earlier version of OpenGL? I haven't been able to successfully query the OpenGL version string for some reason.

Matt