I've implemented a quick-and-dirty version of mouse-based camera control for the battlescape. If you hold down the middle mouse button and drag, it will control camera pan/tilt (in the way common to many other 3D games).
I've attached the patch, but I'm not sure the patch should go into the trunk in its current form; right now, there's no way to turn the feature on/off, control the speed of rotation, re-bind the control, etc. except to change the source code. It may also be brittle; I haven't tested it extensively, and I don't know if it will introduce any undesirable interactions with existing code. If people want to play with the patch and give me feedback, though, that would be great. In particular, thoughts about how this feature should actually behave, and how (or if) the user should be able to control it from the in-game options menus (ie. speed, binding, etc.).