You might want to read this though:
http://www.budgettravel.com/blog/has-the-tsa-stolen-from-you,9797/
Ignore the ignorant author and read the comments, there is the reality you face. Opportunistic TSA Agents who want your valuables.
I might just ship through US Postal service.
Big Mistake!!!!
I had my friend ship an iPhone to me from the US (I am currently in the Philippines on business).
Here's what happened. He shipped it in March, and I received it in June. That's right -- 4 months, even though it was supposed to be here in a matter of a few days! On top of that, it took about $150 in customs fees.
And that's not the worst of it. In order for me to get it at all, I had to have a local employee of mine follow up and make a lot of calls over a period of those several months to get it at all, because Fedex and the Customs office kept telling me about missing paperwork, which was baloney. They had me fill out the same form about 15 times and every time when my employee would follow up they said that same form is missing and it needs to be filled out again.
About half way through all these fill ups I was informed my phone was to be abandoned in favor of the government because of the missing paperwork and me failing to provide it for 60 days (keeping in mind that by that point they had already been sent that paperwork by email and fax about half a dozen times).
After a lot of aggressive phone calls and going up chains of commands in all organizations involved (customs, fedex in the Philippines, and fedex stateside) my phone finally came in, 4 months late with a $150 bill attached which I had to pay before the Fedex man let me touch it.
Apparently, my story is not unique. From googling around for problems with customs and sending Apple products abroad, it seems like many people in many countries share similar tales, not all of which end as well as mine did.
My cost was $150 to ship the phone, and $150 in customs fees, so basically I paid an extra $300 for the $750 phone, for a grand total of around $1,100+when sales tax on the original purchase is included. The same phone would've cost me just around $1,000 in the Philippines (and I could have had it right away) but I thought I would save $100-$150 by having it sent to me. WRONG.
Keep in mind this is just a small phone that cost me $150 in shipping with insurance coverage up to $900. I shudder to think what a 30 pound iMac box is going to cost to ship and how much will insurance add if you need it insured to its full value.
My piece of advice: if you can't bring it with you (and checking it in is a gamble because it can be stolen or damaged in shipping at which point you lose the full value of the item) then just buy it locally when you arrive.
It may cost you a little bit more than buying it in the US but that's only until you consider the hassle and stress of bringing it abroad, the risk factor of maybe losing your entire purchase, and shipping/customs fees which may be added onto you. In the end you are going through a whole ordeal to save maybe $100 or something, assuming the process goes smoothly, and if it doesn't you may be out the entire purchase price of the iMac!
Or another idea is to settle for a Mac Mini. You can bring that with you as a carry on no problem. It's not as cool as an iMac but it is more portable (and cheaper...)