There is a number of very good reasons to fill out and return your Census form, the best of which, in my opinion, are funding and legislative representation. I'm a bit amazed when I see websites advocating refusal, as if it's some sort of government intrusion. It seems to me to be self-defeating to refuse to be counted and then end up having your district lose because of it.
On a lighter note, however...
Hat tip fbihop for the video link