Xbox Live will be down on October 17th for some reason. Major Nelson has stated that this is not the fall dashboard update, but they will be adding some "cool stuff."
You guess is as good as mine on this one, I've got no idea what its all about. Maybe something that has to do with the HD-DVD player?
It is interesting to note that October 17th is not only the release date for Splinter Cell, it is the last day for the PSU demo. What's up with that?!