However, to even gain access to basic information about a user requires you to sign up with their Xbox LIVE Community Developer Program. This in itself wouldn't be a problem, except for the fact that, one, by applying, you are not guaranteed to be accepted, and two, it seems like a very serious application where you must list both a primary and secondary technical contact. I didn't exactly think I was qualified enough to be accepted as an applicant, not to mention that I had a short timeframe to complete the project. So I did some Google searching and stumbled upon Duncan Mackenzie's site, and more importantly, his RESTful API for Xbox Gamertag Data. You can visit his page here.
Simply put, it was exactly what I needed (well, mostly). It displayed the basic user information, including their profile, reputation, gamerscore, and status, as well as some of their recent game history and achievement progress. It would've been a little nicer if it went into more detail about their in-game achievements, but like the saying goes: "Beggars can't be choosers."
While the project is complete, the actual app itself is no longer working because currently, the RESTful API it relied on is, as of April 22, 2011, no more. If you still go ahead and download the .apk file, you might notice a flaw when you run the app. I created the visual layout using absolute positioning instead of relative positioning.