First it will be to catch copyright pirates. Then it will be to identify your marketing tastes -- think adware on the pipe, instead of your client, with no way to clean it out! And finally they will use it to mistakenly identify you as an enemy of the state because you clicked an middle-eastern pr0n banner.
http://bits.blogs.nytimes.com/2008/01/08/att-and-other-isps-may-be-getting-ready-to-filter/
More coverage on the issue:
http://news.google.com/news?tab=wn&ned=us&hl=en&ned=us&q=Network-level+filtering+&btnG=Search+News
