I checked a 21.5 inch iMac the other week. It was in its retail box, inside a soft-sided suitcase (which it only just squeezed inside). Retail packaging took a bit of a whack, but the iMac was fine.

So if you have a hard shell case, you should be in a better place than I was.

That said, I checked it out of necessity and I won't be in a rush to do it again.