I actually hope they are developing a new Mighty mouse quickly for 3 reasons:
1 - It is not very comfortable in the hand and causes hand+wrist pain pretty quickly, and you actually have to learn a new way of holding a mouse.
2 - My fingers get really sweaty, because of the surface.
3 - Pretty often (about 2-4 for me) the trackball get so dirty that it just can't scroll anymore so you have to clean it (Google instructions online) which is kinda annoying to do every time.
Although I really like the side buttons which activate Expose...best feature of the Mighty Mouse in my opinion.
I have been looking at other mouses and I'm probably getting the Logitech MX revolution mainly for ergonomic reasons. A refurb is about €30 on eBay.