The Korean Xbox Web site supposedly has a listing for a Halo 2 expansion pack up right now, and when asked about it Bungie had this to say:
"Next week, we’re going to have a major announcement about the details of our new maps. Right now, there are some dates, details and ideas out there that are actually wrong, so please take everything you read between now and then with a grain of salt. We can't talk about how and when the new maps will be released, we'll leave that to the smart folks who actually do all the logistical work for this stuff – marketing and PR, but we can tell you this much - our entire plan is designed to make new content available for everybody, those with Xbox Live and those without Xbox Live. We hope in the end, everyone will be satisfied."
I'm going to be pretty pissed if they try to make me pay for this stuff. Perhaps even moreso because I know I'll buy it.