I just realized when running it today. In Old Hillsbrad, they reset Thrall's memories to remove the players and the Infinites. I can't remember what happened in Opening of the Dark Portal, but in CoS, sure we were disguised, and Arthas was aided by troops in W3, but there were most certainly not any Infinites in W3.
So, would meeting Infinites have had any effect on Arthas' future? Or did the Bronze's see no need to reset his memories?