Check out the latest video (12.4 MB) (Their FTP s(w)erver is either massively overloaded, or throttled down to nothing, so you may have trouble logging in.) You can put your foot on the thing and give it a big push, and it won't fall over. This is dynamic stability implemented better than I've ever seen before.
The waspy model airplane engine is very obnoxious, but this is just a test platform. A real one might be able to use a silent direct methanol fuel cell to generate power. Of course that increases the IR signature, but if it's using a combustion engine already maybe not by much.
The part about the legs having "compliant elements that absorb shock and recycle energy from one step to the next" sounds verrryy much like Yobotics' Series Elastic Actuators. If so, a very, VERY smart and cutting edge move on the part of Boston Dynamics.
Looks like they're working on exoskeletons, too. (18.7 MB) Damn you guys, using series-elastic actuators in an exoskeleton was my idea! ;]