I don't think there will be any significant functionality changes. If you need a mouse, I would just go ahead and get one. I have both the the Magic Mouse and Trackpad. I switch between them depending on how I feel and what I'm doing, but if I'm using any external device, it's usually the Magic Trackpad.
I hate to say it ('cause I would hate to see it go), but they might just discontinue the mouse. My reasoning is the increasing use of gestures in OS X. Just my own thought, mind you.