Welcome to the 1990s. Microsoft has launched Xbox Live Pipeline, a flash enabled interface to Xbox Live. Of course you cannot download or buy anything, but it is still handy to find out what's new on XBL without have to fire up ( and hopefully not burn down ) your 360. Interestingly enough Microsoft, famous for eating its own dogfood, decided to use flash. For non-Xbox 360 owners Pipeline gives you a little taste of the XBL experience complete with authentic sounds. Yipee.
This is nice to track the download packs to add them to the database... except that they don't list the release date, as expected.
A better place to keep track of the DLC is Major Nelson's site. A really handy resource for mobygamers, because he lists the date the content comes out, how much it's worth, and which regions it's available in.