If the run is long enough, the largest influx of visitors will not be APs or North American Jackson fans, but fans from Japan especially. Michael was HUGE in Japan and, knowing what a favorite stop Disneyland is for Japanese tourists, that could help inflate the number as well.
Also, assuming EO doesn't go back in at EPCOT (a very big assuming at this point), Disney is going to want to keep the show going long enough for East Coasters to take a look. So at the very least, EO could run until the end of summer '10 I would think. We'll have to see ebyond that.