Yes, there is, but in a very limited state. At http://halo.xbox.com sign in and view your career. Then click the Halo 3 stats. Your time played, games played, and overall (social and ranked combined) KD updates. Your game history and things like perfections and exterminations do not. Nor do screenshots or files get uploaded there. It's very barebones, but they may update it after Halo 4's release. It's a low priority for them right now.
[Edited on 05.08.2012 7:08 PM PDT]