i think that the background running apps was just a guideline for the basic developers. I would bet money on the fact that AIM or Apple itself has made an App that will run in the background. Obviously it is possible, look at the iPod app, it is always running.. So either AOL said they would make an AIM app for apple so long as it was allowed to run in the background and Apple said ok (because they are getting a portion of the profit if it is not free, OR they are getting good marketing)
OR apple just decided that ANY AND ALL apps that will run in the background will be made by them and them only. I think this is a good thing because that way we dont have to worry about malicious apps that are draining our battery life and slowing things down.
Like i said, i would put MONEY on the fact that there will be a background running IM application, made by who is the question.