I use 24ga or 22ga cat 5 for doorbells a lot! (Almost always) As I usually attempt the sale of an expensive intercom, and if they bite, I don't have to re-pull wire. I leave one behind the strike too for door release and spares.
http://www.siedleusa.com/home/sbin/siedleusa.asp (For some of those units you have to have spec'ed wire - but most of the smaller "2-wire video units will operate fine with cat-5)
