In case anyone's following this and interested, the "same longitude" thing was a red herring. The real clue was "first time", because i n the real app the first time this zoom happens I also turn on another layer and their markers and other stuff like that. I haven't figured out the details, but delaying that layer creation until the end of the zoom cured the problem. I don't know why only IE9 is unhappy with creating the layer while zooming and not other browsers, but anyhow that's the area where the issue lies.
Rex Bradford
Mary Ferrell Foundation