In my games, I have one DM, so automatically I expect him to know something about some of the monsters we face. Further, since (between several games) we have multiple high level casters (divine, arcane and Psi), there is a great deal of access to summoning spells and planar allies.
I'll be honest, I don't have time to figure out how many hit dice and which feats a summoned Elder Fire Elemental has, and then keep reminding the PC of it's abilities, or control it as a summoned NPC the whole time. I trust my players not to get silly with it. If I want to confuse them, or knock them out of their metagame mode, I follow Piratecat and Monte Cook's examples, and scratch off the serial numbers or apply a template. When you don't know what a creature is capable of, it becomes much scarier. But at the same time, I want seasoned adventurers to feel somewhat capable about battling some monsters.
Case in point: Saturday night we had a game. The first fight was with a half-dragon/dire bear and a CR20 half dragon kobold fighter. The big battle from the previous session had been with several classed Efreeti and half fire-elemental Paragon Beholder. Two games previous to that, the players had encountered a set of 'black snow' wolves...similar to winter wolves, but different enough to make them worry. The Mass Haste that got spent in that battle would not have been used, had they known the quantity of their foes. 3E has made a major difference in monsters, for me, since they've become a toolkit, and not just stats written in stone.