This behavior can be very a normal developmental phase that some children go through. This makes sense when you consider that children of this age often lack the verbal skills to communicate what they want, or to negotiate situations. Mix a few kids of this same age together and you have the perfect breeding ground for aggressive behavior.
Most importantly, just because the child has engaged in aggressive behavior, does NOT mean that the child is aggressive or violent. I would be very cautious about labeling such a young child as aggressive.
When children use aggression to get their needs met, they are telling the loving, caring adults in their environment, "I need YOUR help. HELP me to figure out a better way to get my needs met."
Adults who work with children in this age group should be well-versed in the develpmental tasks of toddlers and preschoolers...and view their role as "coaches" and "supporters" for children who exhibit challenging or aggressive behavior. Rather than saying, "Here we go again...Joey can't keep his hands off of the other kids!"...we could be saying..."Uh-oh, looks like Joey needs some more support to figure out how to get his needs met in a more acceptable way."
I have just written a more extensive article on this topic, "HELP! My child is being aggressive!"...which you can find here:
http://www.examiner.com/x-31855-Upper-Peninsula-Family-Examiner~y2010m6d6-Help--My-child-is-being-aggressive?cid=examiner-email
Remember this: Some kids need more help than others when it comes to learning colors, shapes and how to tie shoes...and some kids need more help when it comes to learning how to get along with others. It is our job as adults, to make sure each child gets exactly what he needs in order to succeeed in both academic AND social realms!
Good luck! I'm sure this can be worked out!