I recently acquired another 23” display. Whilst this new screen looks superficially the same as one of the existing units, it clearly has a different LCD panel. Rather irritatingly, the new panel reproduces colours quite differently to the older one, and the Apple supplied ICC profiles really don’t do much to fix this.
Calibrating the two using the software calibration feature in Mac OS X didn’t help that much. It’s just too hard to get accurate—or even reproducible—results by judging colours with your eyes. If you only have one display, and don’t otherwise care about colour, you might not have noticed this, but believe me, with two it’s a real nuisance.
Anyway, I invested in a gretagmacbeth Eye-One Display 2. It’s made a big difference. Both displays now appear basically the same; you can still detect a subtle difference in between the two (due, I think to the native whitepoint difference), but it’s a whole world better than the default settings.
One other benefit of having a proper calibration tool is that it’s possible to get the two panels set to the same brightness, which is very helpful (again, it’s quite hard to judge if you’re trying to set it manually, although you will spot the difference during use, which can be annoying).