To my knowledge, Microsoft is generally very close-guarded about Xbox 360 title updates in general. More likely than not, a large percentage of them are security and stability updates. The best resource I've found thus far to keep up with major title updates is the blog of Major Nelson. He's the de facto "head Xbox spokesman" at Microsoft, and seems to generally try to comment on title updates when he is able and has useful knowledge to impart. Definitely when there is news about a title update bringing big new features, Major Nelson has been amongst the first that I've seen to announce them. Major Nelson's podcast will also sometimes feature comments on title updates.
Beyond that, often individual publishers and developers have different PR plans. If you track down individual game websites, sometimes they will comment on their own title updates and what to expect from them. Often these tend to correspond with DLC releases, and so can often additional information on title updates is sometimes buried in PR releases that are sent to news sites about a game's DLC.
I don't think anything more comprehensive exists, and I'm not sure it would be allowed to exist. Certainly few gaming systems can be as usefully informative about game updates as Valve's Steam often tries to be.