Can Mobile Devices Replace Laptops?

The question is a good one.  If they can it will saves millions of dollars for businesses as they will not need to equip their mobile workers.

A second question is what will a mobile device be; a netbook, an iPhone or like device, or a regular cell phone?

Can you use current applications or do they have be rewritten to be used on mobile devices?

Is it possible to have a single device or does practicality make it  necessary that we have at least a phone and a display/input device?

We are looking at these questions and want to hear what your thoughts and experiences are with business and mobile workers.