> "My dream productivity device is a modern take on a Psion 5MX."

Adapt it out of the conventions of the inferior form factor by making it a detachable, i. e. a UMPC in a smartphone-like form factor, and you might have a winner... if you don't skimp on all the other good stuff that makes a great ultra-portable general-purpose computing platform.

> "Thinking about it as I type this, perhaps a psion-style keyboard for an iPhone might hit the spot [...]."

The problem with the iPhone is that it's not a UMPC, but locked-down crap.