I'm looking at the new 2.8GHz 24'' iMac.

But... I will need to bring it on a plane, preferably carry-on. Does anyone have any experience with this? Or using the PROi24 iLugger case?

It is likely to be too big for most carry-on restrictions once you put it in a suitable case (or even the box it ships in.)

Measure the case and then check your airline's website for restrictions. If you are flying on anything smaller than a 737 the restrictions are even tighter.