C.,
My experience is that kids definitely go through hitting stages, even more so if they're around other children who hit, but it's also just a natural reaction for some kids.
I'm an older Mom (so I've got a more old-school mentality - so please take my advice with a grain of salt) :-)
When my son went through that stage, I bought him one of those inflatable toys (like the incredible hulk, etc., even one of those blow-up easter bunnies will work) and told him this was his punching toy, and it was okay to hit this toy - but never anyone or anything else. I think it's really kind of a 'boy thing'. Sometimes boys just have to act in a physical way, where girls tend to whine and cry about things. We went through a few of those blow-up toys and later he graduated to a small punching bag with kid-size boxing gloves in the basement. My son is now 18, and of course - doesn't hit. :-)
My youngest daugher is now 4 1/2...when she hits me, she is immediately sent to her room for a time-out until she is ready to come back and apologize and I let her know this is very serious and wrong to hit any person (or animal). I got her an inflatable toy and told her she can hit that if she needs to hit, but she doesn't do it nearly as often as my son did at that age.
hope that is helpful,