Well, I both agree and disagree with DracoSuave.
For one, I don't think I the dominated character has full power over who is its ally or its enemy - for example, if dominated to hit his friend with an attack, the player couldn't say "the attack fails, because the target is "all enemies", and that's my ally, not my enemy".
How many at-wills in the game exist that have such stipulations on them?
That's the other thing, you're only using at-wills so it's not like there's a lot of corner cases involved.
Additionally, there has been some discussion as to whether dominated creatures get free actions, and who can take them (the creature or the one dominating it).
The dominator. Free actions -are- actions, and the dominator controls those. Ask: Is it an action? Yes? The dominator controls it.
Also, if you have immediate or opportunity actions, -even if they are at-will powers-, the dominator cannot make you use them, because you are dazed.
So, no, a dominated creature doesn't OA.
My own opinion is that it is DMs call, and depends on the circumstances - for example, I have no problem with taking saves (even if free actions), but I wouldn't let him use free actions to negate the dominator's use of his actions.
This is why free saves tend not to be free actions.
Additionally, there is also debate on who decides whether one is an ally or an enemy for the purpose of the rules, as it isn't laid out in the rule books.
Sure it is. If they are a willing recipiant for your power, they are an ally. This implies mutual consent. All other cases are enemies. It's pretty easily laid out.
For instance, a cunning bard can slide a missed ally one square - if the bard get's to choose whose an ally, he could slide a missed monster 1 square, which potentially could have very nasty consequences or merely be tactically very powerful.
I doubt the monster is allowing this use. Therefore I doubt he's your ally.
Now, if this turns out to have an unforseen ramification, then yes, the DM should rule against that particular case, but that's not a hole in the ally/enemy rules, that's his job and applies to -all- rules.
Regardless, dominated doesn't change the enemy/ally mechanic. It ONLY allows control of your actions, and not a single bit more. Otherwise, you could never Font of Life out of it, and other such things.
Given that, unless you're fighting Grazz't, the enemy/ally thing won't even really -be- much of an issue, seeing as you're only using at-wills, the only thing left is if you have an ability that benefits a target ally if you hit.
Not to mention Grazz't has to -specifically include- text that states you switch ally/enemy relationships unlike other dominating powers in certain key situations (flanking, IIRC)....
And yes, that can only target one of your allies, so chances are the monster will not do that.
All things told tho, probably the most -powerful- thing to do with a dominated creature is have him take a run action through a gauntlet of OAs. That'll probably do -far more damage- to the enemy's team than an at-will will ever do. Especially if it's a ranged attacker.