If we were to design a UMPC, making it small enough to put into our pocket is pretty much the primary feature we'd focus on. Amtek has the same idea, which is why their U560 prototype shown at Computex actually looks promising.
It's a flip-up UMPC with a full Sidekick-like (sold in AU as HipTop) QWERTY hidden underneath the closed screen. It's got an Intel 600MHz/800Mhz processor (depending on the version), a 1.3-megpaixel camera, 3600mAh battery and apparently runs Windows Vista.
There aren't very many other details, but if they can really put out this device with this form factor, we're sold.
New U560 UMPC from Amtek [Gottabemobile]