FWIW, I use PM on 1024x600px screen subnotebook (DIN A5 size) almost daily, and it's manageable. Some dialogs are not fully visible (Save As), but one can get around that somewhat. Other functions work just fine. And the subnotebooks' small size is a big bonus.
I would be a bit wary of the UMPC devices though, try a real small subnotebook instead. Something with real Pentium-M or Core Duo processor and enough RAM. As a bonus you will have better battery life usually, normal (though small, but not stripped) keyboard and all the usable ports (most have FW, USB, some PCMCIA).
Frantisek