Had an interesting play happen the other night.

My opponent was playing a Night's Watch deck. He played the card "Take the Black" which says, " Dominance Action : Choose a non-unique character with printed cost 6 or lower. Take control of that character. (Max 1 per round.) "

He used this card to take my Hedge Knight. This Hedge Night had a "Mare in Heat" attached to it.

My question is, what is supposed to happen to the attachment? Does it get discarded once my opponent takes control of the character? Does the attachment stay with the character, but still under my control? Does my opponent also take control of the attachment?

We decided that the attachment stayed with the character, but my opponent could not kneel the attachment for its effect. No idea if this was the right call or not.

Control of characters and attachments is determined separately for each individual card. The player controlling the character does not have to control all attachments on it. This is probably clearer when you put Milk of the Poppy on your opponent's character than in the situation you describe, but it's effectively the same - you can control an attachment that is on a card you don't control.

So, since "Take the Black" doesn't say your opponent takes control of any attachments on the character he steals, he does not. He takes your Hedge Knight, but you still control the attachment.

This has wider implications, some of which you already mentioned. The attachment stays on the character your opponent steals, so any continuous effects (think of Little Bird giving the character the INT icon) remain on the character. However, since you can only trigger abilities on cards you control, the person who steals the character cannot trigger anything on the attachment. Interestingly enough, the controller of the attachment CAN.

So, in your situation, you did exactly the right thing. Mare in Heat stayed on the Hedge Knight, but the Night's Watch player who stole it could not trigger the attachment's ability when the Hedge Knight attacked or defended alone. Interestingly enough, YOU could have still used the attachment -- although you'd have no control over when the Hedge Knight attacked or defended alone.