The Apple rumor site MacBlogz is getting a lot of play from some mockups they created of a mighty mouse augmented with a touch surface on top for scrolling and gestures: One More Thing: Apple's New Multi-Touch Mighty Mouse.
Apparently this is based on some Apple patent applications (which I haven't looked at) and not just speculation.
This wouldn't be the first mouse sold with touch sensors for scrolling. The Logitech V500 mouse (pictured below) has touch scrolling and it works quite well.
So scrolling makes sense, but could you really do other multitouch gestures on top of a mouse? I can imagine two-finger tapping or two-finger scrolling mapping to different functions, but the main multitouch gestures Apple has given us so far — pinch, flick, and rotate — seem like they'd be too awkward to perform.
So I doubt Apple would do this. More likely, in my humble opinion, would be a new keyboard with integrated multitouch trackpad. After all, why do you still need the mouse if the trackpad is good enough? Lenovo sells a thinkpad-style keyboard with pointing stick and touchpad (see Design Matters). I could see Apple doing something similar.
Also more likely from Apple, in my opinion, would be a large gesture surface or keyboard similar to the old Fingerworks products.