Perhaps frames might help. The list of links to the left is a lot of overhead on a page like this. It probably wouldn't look as good as it does right now though.
I too have spent some time working on CSS and XHTML in order to reduce the bandwidth consumption. Unfortunately this stuff has had to take a backseat to some other projects but I hope to get back on it soon. Initial indications are that we can save 25% - 40% of our bandwith by making the switch.
At one point in time the menu was a separate file that was cached locally on a user's computer so they didn't have to fetch it with each page. That worked well for about 95% of the users. We eventually switched away from doing it that way for various reasons and never really considered doing it differently.
I'm not sure what out theorethical max bandwidth per day could be. I supposed I could figure it out but I'm lazy. We do hit the max from time to time for an hour or two per day. If you're ever curious you can always look at our bandwidth monitor. The numbers reported on the monitor are for the entire circuit, not just ADT, so they are higher.
They didn't go the whole hog, though, and they also heavily rely on Flash, which is never a good idea. They might have saved more if they went for minimalistic design. Too late...