Yes, you can include the iMac as a checked bag. Do check with the airline to make sure, but there should not be any problem.
It would be best if you can pack the iMac in its original box with all its original styro packing. And then put the box inside another, unmarked box. You can get empty boxes from OWC and other vendors. And, as @VikingOSX already told you, insure it for full replacement value for the trip.
One final note, also check the customs requirements for the country you are traveling to. It may be necessary to declare the computer (either due to its value or it just being a computer) and possibly pay a duty. It depends on the country.
You should also check US regulations about exporting a computer and/or any encryption capability. There are certain countries to which the US bans such exports. I suggest checking with the State Dept. and the Office of Foreign Assets Control. This is not to scare you, just suggesting that you cover all bases since you didn't tell us the country you are moving to.