It looks like the ZR30W uses a fluorescent backlight. Although it's a cold-cathode fluorescent, the color still changes a little with the temperature. You want to be sure you let the display warm up for quite a while before profiling it to be sure the temperature is stable. The usual recommendation is something like 20 minutes as a minimum, but from what I've seen an hour is considerably better; if you have really good color discrimination, two or three hours wouldn't hurt.

I should also point out that some monitors (even some that otherwise seem pretty "high end") don't seem to have very stable backlights no matter how long you let them run. I don't know if I applies to your HP monitor or not, but (just for one example) at work I used to have an Apple Cinema display. I could run it for eight hours straight, profile it, and then profile it again 20 minutes later, and both the color and the brightness were quite noticeably wrong. OTOH, even though it also uses a CCFL backlight, I have a LaCie 321 that's so stable I can actually measure seasonal variation. During the summer my office gets hot enough that its temperature shifts a bit, but when I re-profile it again in the fall, it goes back to where it had been a year before.