I'm almost certain that Itachi does use hand seals. It's just that he's so fast it's hard to catch him at it. In his fight with Kakashi Itachi pulls out some weapon with one hand and does seals with the other. Kakashi makes some kind of comment about how the weapon was a distraction and the hand seals were so fast even he couldn't follow them.

I guess he can do hand seals with one hand, and that's supposed to be difficult.