It's technically possible... if you're willing to do some major, major surgery.
I've replaced hard drives on pretty much every mac model over the last 7-8 years. The white 20" iMac (second case version) was the most difficult by far. The new aluminum models adds to the difficulty of that one by forcing the user to remove the glass screen (most guides I've seen involve using a suction cup device). I'm not willing to try it, and I'm willing to take apart almost anything.