Withholding information about damage reduction seems to me to be a mean trick on the DM's part. The DM is the information bottleneck of the game; if he witholds information from the PCs, there's no way for them to realize what's going on in the game. Now, I'm not advocating telling the PCs flat-out "Your slashing damage is being reduced," but if the DM doesn't tell the PCs that their attacks are, in some way, being reduced or negated, the PCs don't really have an idea that's going on in the first place. To them, they just assume that their blows are landing as solidly as they might against any foe, and that this particular foe just happens to have a lot of hit points. A PC who's told, after hacking away at a foe for 10 rounds to apparently no avail, that his attacks "aren't hurting the monster as much as he might think" has been decieved—his CHARACTER would have and should have noticed the lessened effect of the attacks from the first.
It's best, I think, to describe a blow that is reduced by damage reduction as just that: "Your attack hits the monster, but doesn't seem to hurt it as much as you thoght it would," or something to that effect. You don't need to tell the PC what part of his attack didn't work; leave it to him and his Knowledge checks to determine if he needs a different material, a different magical augmentation, a different type of damage, or whatever to get through the monster's DR. But if you don't tell him, he won't know. That's the DM's responsibility; to let the player know what his character knows and experiences.