Hi David. Just want to add a little to what's already been presented -- with apologies in advance if this is ground you have already traversed.
First, it appears to me that Apple has changed something in OS X's overall handling of color that makes it slightly more "Disney" and noticeably less natural (as in smoothly graded across saturation and luminance transitions, and weighted towards the perceptual centers little clustered on the edges). There is not much you can do about it (if it even is true). If you use Aperture, you should consider saving a custom setting in the "RAW Fine Tuning" Brick for each of your cameras. I recently purchased a Sony a77; after a week I significantly lowered the "Boost" and the "Hue Boost" in my default for that camera (this was not needed for the a900/a850). Similar to all monitors being sold with a default brightness that is too bright for color-critical work, it _seems_ that Apple is producing software with default settings that are too saturated (and too contrasty) for color-critical work.
Second, the iMac is not a "good" monitor for color critical work. I suggest borrowing a monitor that is good (the better NECs, etc.), hooking it up as a second monitor, calibrating it with your X-Rite device (which one?), and then comparing the two monitors. Be sure to calibrate to the same cd/m^2 setting. I use 85, which gives me the best correspondence from image to print for my equipment and environment. HTH.