It's all on BD discs. 360 games are on DVD with no games ever intended for the HD-DVD add on. It will be quite a while before any games come out that stretch the 9gb DVD size barrier.
And even if it did, just throw in Disc 2. This is one of the arguments for BD that I never understood. 50 GB is great but so is two 25 GB discs. If a movie is that long to go beyond 25 GB, then I need a break to grab a beer and a piss anyways.