The mouse is STILL the best, most practical/productive interface yet. Will something better come along? of course...but that something CERTAINLY is not your fingers, using multitouch holding a 10 inch screen (can you say fatiguing?).
It's funny because in the past few years, I've spent 99% of my time on a laptop with a trackpad. When I go to a desktop now, I find the mouse really fatiguing. I constantly have to move my wrist and arm around, instead of moving just my finger. It's all what you're used to. Stop using a mouse for five years, go back to it, and tell me if it's still the best thing ever.
Is the iPad more fatiguing than a mouse? I won't know until I've used it. However, it's definitely a more intimate experience. It's hard to describe and I felt Steve struggling in his keynote on how to express this too (the awkward, "feel the internet in your hands" line)
When I use my iPhone, I feel like I'm controlling the device directly in a natural and intuitive way whereas when I'm using a PC, it's a more passive experience. I'm controlling a mouse/trackpad and that mouse/trackpad is controlling the PC. Technology is getting in the middle of me and my content.
As far as productivity, there are a lot of tasks in which the iPhone interface would be a lot faster than a mouse. Punching in numbers(Calculator), scrolling through things like contacts, music (iTunes), photos, movie times, etc., moving objects around (calendar, spreadsheets and presentations in Numbers and Keynote, video and photo editing), and even just clicking icons, buttons, settings, etc.