The Roam Only option does seem to limit the Data Roaming to "1x". If the "automatic" option is selected then Data roaming seems to work fine. But of course that comes with the same problems of constantly being in and out of spotty Sprint coverage. When "Roam Only" is selected EV-DO data essentially is eliminated as a data option. Maybe someone here can figure out if it has anything to do with the Pre itself or some kind of network controls.
I used to Roam often (forced and unforced) with my Sprint Touch Pro and it would always default to Alltel's EV-DO data. Not sure what the difference could be between the Pre and the Touch Pro when it comes to handling forced roaming.