Hang in there Robyn, my son is the same. He got belted by a child at playgroup and the rest is history. He was throwing things long before then, and pity for us he is a CRACK SHOT.

Sorry Elli - but talking doesn't work with a 2 yr old! It's generally the fact that they can't communicate themselves that leads to the frustration and lashing out - especially with boys.
Anyway - he probably does want a reaction (it's a sure-fire way to get one!), I find it best to get DS busy and distract him if he gets that look in his eye. I also find ways to praise him when he is good or if he stops himself from hitting so he hopefully will realise that positive attention is more fun than being told off.

There is a great book called Raising Boys by Steve Biddulph, I'd recommend you have a read, I found it fantastic.
Yes, it is embarrassing, you just have to keep on top of it. Lots of kids hit/pull hair/take toys etc, but if you are on top of it most parents don't seem to mind (I think!).

Good Luck!