The iPad 3 pushes 2,048 by 1,536 pixels. A single monitor 30" requires 2560 x 1600 (or maybe less, if it's crap). So ... I can see an iPhone being able to drive a single 30" monitor some time soon.
People will be using their phones as desktops (if not serious workstations) sometime soon. And once they use them as desktops, phones won't be fast enough until they have performance comparable to workstations (which, as you point out, won't happen).
Why not? If it doesn't melt while being maximally used in my hands, why should it be thermally impossible to put it on the table while connected to an external screen?
The processor is more powerful than many old computers, and the 30" screens need no more pixels than the iPhone screen already has.
I wonder if it would be practical couple the CPU directly to an externally accessible thermal pad, and have some sort of docking station which includes additional cooling. There are probably better cooling options available (liquid, heatpipe?) if you could find a way to connect/disconnect them reliably and not compromise much on the mobile aspects of the design.
Yes, it would be very useful to have a single OS image that could scale its user interface and capabilities to the hardware it finds attached at any given moment.
OSX already does this. Got a base model Macbook Air and a big powerful iMac or Mac Pro? Connect a thunderbolt or a firewire cable and boot in target disk mode, you'll boot from the Air's hard drive but get the full hardware capabilities of your iMac or Pro (better GPU, more CPU power, whatever).
People will be using their phones as desktops (if not serious workstations) sometime soon. And once they use them as desktops, phones won't be fast enough until they have performance comparable to workstations (which, as you point out, won't happen).