[Mr_Sandman]

For human opponents, I usually let players know the DR, since characters can see the armor, unless there is some magical or other hidden protection the characters would have no way of knowing about.

For previously unencountered creatures or human(oid)s with secret protection, I keep DR secret at first. Once the PCs get in a couple of hits that actually penetrate, and can see the results, I often let them know the DR number so they can help with the number crunching.

For something obvious like zombies, I would be open with letting players know they have IT: Unliving. For less well known creatures, or a vampire that they don't know is a vampire for example, I'd probably keep that to myself, and do the math for injury tolerance in my head.

I don't announce opponents' stats, but I roll in the open, so if a fight goes on for a little while, players start to get an idea of the opponents' skill and defense levels.