Interesting thread. My friend Amy has been thinking about this issue with her son, who is not yet 2. A few months ago she felt he was being the "bully" with other children at daycare--it sounded like he was a little bulldozer, and just crawled over other kids or pushed them aside, and maybe took toys away from them. Now she is concerned that he is being bullied, by a girl who is younger but bigger and more aggressive. She and the girl's mom have been working to teach their kids to be more assertive/more considerate of others, respectively. I'm not really sure what that translates to, practically speaking, with children under the age of 2.
Amy is sensitive to the issue because both she and her DH were bullied as children (teenagers). For her it's important to know that the other mom is working with her daughter to change her behavior, and they have probably discussed exactly how the other mom is doing that. These things are happening at daycare and being reported by the daycare worker, not witnessed by Amy or the other mom, and presumably the daycare worker is dealing with them in some immediate way that both moms agree with.