Hey for the past few months I've been looking for the same thing - a quality pair of headphones with a mic to use on my Xbox 360, and PC. I'm glad that I found this site/thread cause I have almost the exact same situation as you Shazam!
I was originally looking at TurtleBeach stuff (cause its cheap and I'm poor atm), however I have a pair of Sennheiser HD595s. I love these headphones - I got them for school for audio class (sound editing) and use them for gaming as well (only single player though). I also bought a Behringer Xenyx 502 (5 channel mixer) so I could have both PC and xbox audio come out of my stereo.
Now like you Shazam I thought of finding a mic to attach to my headphones, but didn't want to ruin them or always have a mic attached. I was also looking for a way to control the mic volume and so looked up the Astro Mixamp - which eventually led me here. However I just realized that "hey! I already have a mixer!" so why would I need to spend another $130 on another mixer?
Assuming I get a mic (I really like the Hyundai one used in the mod Rebel posted) and attached it to my headphones (adhesive seems cool, but I'd prefer a clip), do you think I could rout it properly through my mixer? The Xbox controller needs 2 way signal to receive the mic input and output the chat correct? If so is that where the Astro Mixamp would come in handy, cause I don't think I can rout that with my mixer.