Subject is pretty clear, do you let your players know, either by explicitly stating it or by describing them in a certain manner, that a certain 3 (or 4) npc's in the encounter are "henchmen" and subject to all the limitations thereof?

I hated the 1 hit point henchmen monsters in d&d4e because I always felt bad when a player used a big attack on the 1 hp guy and wasted it - but at the same time I hated the idea of telling them it since it seems so metagamed. At least henchmen compared to regular dudes in wfrp are less black/white so it's not so much of a mechanical problem.

Kaptain O said:

Well I think the difference is a henchman squad is going to be more than 1 so they will most likely get the idea where as 4E one minion "looks" like one regular monster.

Sinister said:

Yeah, and when they get too comfortable with that, throw 3 individuals that advance as a group at 'em and hose 'em.

I will normally just describe the leaders and non henchmen as bigger and meaner while the mooks just get the "its a beastman" treatment my players quickly caught on that not much of a description meant easy kill.

I usually only have one mini/standee per henchman group, so it is pretty obvious. Plus, this way the PCs can give me wonderful descriptions when they kill 3 guys in one attack roll. E.g. I kick the front man down, turning to bury my axe into the belly of his friend. As the skinny, cross-eyed guy closes the gap, I duck under his blade and take his legs out from under him. Raising my axe, I bring it crashing down on his head, and then move to finish off the first man who's just starting to rise."

What were your PCs fighting that they could kill 3 in one attack?

I did that as the Slayer during the demo, but it was only because the GM wasn't subtracting the enemy's Toughness from the damage.