The Boy Genius Report just posted a great comparison of the Vulcan FlipStart, the OQO Model 02, and the Sony UX280P. He compares nearly all facets of each system, from design and portability to feature set and configurability. If you are trying to decide on a 4" screen device, this is a must-read.
I’ve used all three myself, although I’ve never had the opportunity to compare them directly. My favorite is the OQO, hands-down. Small Tablet PCs like the OQO are hard to find, and the active digitizer is even more rare. The biggest point in the OQO’s favor for me – it just feels good in my hands. The tactile feel of the unit is just a pleasure for me. The Sony would come in second, but it felt oddly thick to me. Personal preference. The FlipStart was a non-starter for me. No touch screen, no digitizer. It’s just a tiny laptop.