The main problem would be, as Hello Panda! noted, space. Unless you're absolutely certain of the dimensions of your room, you might have a problem fitting it once you get it here. If you find a place ahead of time, and can trust your estate agent/landlord to accurately measure, and you're as big a control-freak as I am, you could draw a floor plan to scale, and then play around with different layouts to make sure you've got room. Just be sure to account for things like closet doors (if you're lucky enough to have any), radiators, outlets, etc.
On top of that, you need to think about the shipping cost. If you're bringing a lot of other furniture and stuff, and you'll be getting a whole/half container anyway, then why not (as long as it'll fit)? But if everything else you're bringing would fit in boxes on pallets, then whatever extra you'll pay for a shipping container might very well cover the cost of a brand-new mattress. A few years ago, we bought a top-of-the-line Sealy Posturepedic queen/king mattress and bed for around £450 total, with free shipping (and that's really impressive given our location). You just have to be prepared to shop around.
There's also the size difference problem. A lot of your platform-style beds (like a lot of the IKEA ones) shouldn't be a problem, but a divan (box spring w/ built-in frame/headboard, or anything with a foot-board, or an even slightly-sunken platform/slats (i.e. where the side & bottom edges come up around the mattress a bit, to keep it in place), and you could have a problem. At the least, you'll have to pay careful attention to the dimensions. You'll have to think about bedding, too. Having fitted sheets that are 2" too long (US bedding/UK mattress) is merely an annoyance, and can be solved pretty easily. Having them 2" too short (UK bedding/US mattress), means you're looking at a bare mattress. While you could simply plan to only buy bedding in the U.S. (I agree with vadio re: cost/quality), you have to ask yourself, is it really worth the extra cost (or, alternately, aren't there a lot of other things I could stick in my spare suitcase on the way back from a visit home)?
I guess the bottom-line is, unless you're certain it'll fit, and it's either a super-expensive custom mattress, or you're already shipping a container full of stuff anyway, then it's probably not worth it. And if you do bring it, stock up on sheets/mattress pads/etc. before you leave, and be prepared to shop carefully (and with limited selection) for your bed.
