I'm playing through HeartGold right now...
Wondering, though, with roamers in 4th generation, does the nature of the monster change each time that a battle is initiated or is it determined the first time that you encounter the roamer? Main reason I am asking is that I've got a synchronizer for an adamant nature that I want Entei to pick up, but without a master ball it is inevitably going to take me several battles before i successfully catch entei. If i could synchronize his nature one battle and then put a poke in the front of my party to stop his fleeing (like wobbuffet), it would definitely make things easier.