I read about the HUD Color Picker in a description of what's new in CS5, so decided to try it. The HUD did not display with Shift-Alt-right click in any of the tools it's associated with.
OpenGL is enabled, all the OpenGL functions I've tried like dynamic zoom work as described. The image of the brush being use is inset in the window when the Mixing Brush is selected. But no HUD.
Pretty much by accident, I discovered that after an Alt-right click drag to change size or hardness of the brush, the HUD begins to work. (You have to do this in Brush, not Mixing Brush.) And continues to work on any image opened before closing Photoshop. Broken again after restarting the program.
Classified now mainly as an annoyance now that I've discovered a work-around, but I don't think this is operating as designed...
Windows XP SP3 AMD Athlon X2 3GB RAM Photoshop 12.04 X32 Radeon HD4830, latest drivers