Is Everything Color Managed in OS X?

If I understand your question correctly, when you calibrate the display, the calibration applies to everything (even including the content "inside application frames") as a final step -- it affects the entire screen. So OS X wouldn't really look different on the two different screens as long as they are both properly calibrated, to the extent that the displays are capable of the same color spectrum and contrast. Uncalibrated, of course, it would look different on the two screens.

 

I'm not sure if it's fixed, but definitely in 10.4, the way certain applications handled colour profiles was different, particularly for external monitors.

For example, 'basic' apps, such as iPhoto, would apply the 'primary' colour profile for the display only, whereas 'pro' apps, such as Aperture (and Preview actually too) would apply the correct colour profile for the display. You could see the difference if you opened an image, and dragged it over the two displays - the image would change colours in the proper app, but in iPhoto it would not change, and look weird on the monitor. I discovered it that the main profile was loaded during log in, and this was kept and used.

I haven't really used an external monitor since I got 10.5, so I don't know if this was fixed.

 

Everything is colour managed.