Very simple, you fly off of one island, it gets fogged out, unload that island. When you get close enough to a new island that island/map is loaded then fogged in.
But what about load times?
Well if worse comes to worse we'll invest in threading, but I don't see that happening, especially since OGRE is looking to support threaded resource loading for models and such, it doesn't take long to load the terrain =P.
