A buddy of mine who goes camping carries a foam eggcrate-looking thing with him. He explained that it's to lay on the ground underneath your sleeping bag as a measure of insulation. Even rolled up the thing's pretty bulky, but he says it's indispensable.
The AeroBed Pakmat seems like a neat alternative, and I dig the design (though I should admit I have very little camping experience). First off it's inflatable, so air pockets provide the insulation rather than bulky material, and once emptied it rolls up to a fraction of its size. Secondly it stows away in the canister you see here, which is actually the hand pump you use to inflate it.
You can be fooled by the product photos into thinking the canister's smaller than it is, as it kind of resembles a water bottle; but poking around the web I found this review, and the photo below shows its actual size:
The $120 asking price on Aerobed's website seems kind of steep, but they're also selling them on Amazon for 96 bucks.