If I plug headphones into a reciever that is outputting dolby digital 5.1?????

As far as I know most receivers only output two channels through the headphone jack. Best way to check is just plug it in and find out. What kind of receiver do you have?

No.  It either down converts to stereo or it only plays the front stereo channel audio output depending on how your receiver works.  I think there is a few receivers with Dolby Headphone or other like headphone surround virtualization but it's rare.  I recommend that you put the 360 into stereo mode when you want to use headphones.  It'll send out an uncompressed PCM 48kHz signal over the optical cable rather then Dolby Digital 5.1 AC3 compressed audio (it also does the better down converting than the receiver).