But unless the power specifically says so (see assassin's Shade Form), ally cover doesn't count for hiding. You can't maintain being hidden behind an ally (full cover->hide->move behind ally won't keep you hidden.), thief's trick doesn't let you hide with cover from an ally, unless it says so specifically. Why? Because the rules specifically say allies only give cover when determining ranged attacks. Enemies aren't ranged attacks, neither is seeing, so they can see you just fine.
But, if the power says you can use allies as cover, then there you go.
Edit: Checked the book, it make no mention of using allies to hide, therefore you can't