Actually I guess it's not enabled by DEFAULT, but there is a cvar: mp_friendlyfire 0/1
http://wiki.teamfortress.com/wiki/Friendly_fire I guess there's some weird exceptions of things that don't do damage. Also it probably is buggy as hell.
Heh, I remember this one time I was on a server and an admin turned on friendly fire so everyone could help kill a blatant hacker. Fun times :fyeah: