I keep a ThermoLite 2.0 in my bag and love the thing. It's basically a space blanket that's on steroids. It's a lot tougher than a regular space blanket, not as noisy, and it's made into a sleeping bag.

As stated above, condensation is an issue, but it has venting to help cope with that and it's very warm for a sleeping bag that packs the size of a Coke can and only cost me $30 or so.