Since it actually makes this at-will power very useful (for two classes) and it does require some tactical play to set up, I'd say it's pretty reasonable. Sure, intelligent creatures may not fall for it, especially not more than once, but I would think undead and animals would get smacked a lot. And even if something doesn't fall for it, the worst that can happen is you have a useful at-will power.
Thanks for the opinions!
It works exactly as everyone has said. I know someone playing a Warlord who attempts to set this up nearly every combat. But he retrained out of Brash Assault since the character was in LFR. In our city, we have a core of 8 or so DMs. He played games mostly DMed by one DM who refused to take the attack ever by anyone.
I believe the conversation went something like this:
Warlord: "He gets a free attack against me with Combat Advantage."
DM: "But he's marked to the fighter, right? So the fighter gets a free attack against him twice and you get a free attack as well if he takes it?"
Warlord: "Yeah, but he doesn't know that."
DM: "Sure, he does. You just used the power against him and he knows the full text of any power used on him, as per the rules."
Warlord: "Sure, but it's a wolf. It's not smart enough to make that tactical decision."
DM: "Actually, it's precisely BECAUSE he's a wolf that he won't take it. He knows he is marked at an instinctive level and won't attack anyone but his mark."
Warlord: "Sure, whatever."
Then repeat that same conversation again when he used the tactic on an intelligent enemy. Only this time, the DM said that the enemy was too smart to fall for it.
Most of the other DMs he's played with have taken the attack...once, then won't take it again for the rest of the adventure. Once the DM figures out how powerful the combo is, even monsters in encounters who have never seen the power before decide not to take it.