i-mate plans to create a “shell” that would feature a 1024×768 display (7 inch?), a keyboard and a battery that can last (up to) 80 hours, turning the Ultimate smart phones into a comfortably sized PDA powerful enough to do serious productivity work, yet much smaller and lighter than a laptop. If you need to, you can also remote control your home/office PC with Remote Desktop.
However, there’s a price to this. Expect the “shell” to cost around $300 in addition to the $600-$800 of the Smartphone itself. It’s a little expensive, but for someone who mainly does email and who really cares about portability, that might sound pretty good.
Update 1/2/08: Take a look at the REDFLY, it’s a mobile companion that works exactly like this.