Wal-mart $20 air mattress is the way to go.
Sleeping on 6 inches of air beats sleeping on 1.2-2.5 inches of air/foam.
Plus, a twin air matress is wider and will pack smaller than a comperable thermarest.
Yes, you open up the possibility of a leak and sleeping on ground but that's only happened to me once and I just got another mattress so I'm out $40 instead of the $120+ for a thermarest.
Actually, I have a thermarest that I probably should sell, after this post, I doubt anyone would want it... LOL
If you can, try to find a very small air pump, K-mart used to sell them for $5. You can put a bmw plug on the end of it and plug it into your outlets, like I do.
The worst sleeping I've ever done camping was on a thermarest, the best was on the air mattress.
Important note: Don't get one bigger than you need, if you have a big tent and you get the 'full' size, if it's cold, the cold air will pull the heat from your body. with the Twin size, I've never felt like the mattress was pulling my bodyheat and I've used it in 30-40 degree weather. In fact my upcoming trip to Big Ben next week might touch on the mid 30's to mid 40's as a low, I'm not worried about a good night's sleep.