If someone walks up to me, asking me, "What's that?" I need to express what it is in a single phrase that they will understand. If I say that it's a handheld gaming computer, they'll probably ask if it can run World of Warcraft. Unless I'm with a bunch of computer geeks, no one will understand if I say that its an OMAP-based handheld gaming computer that runs Linux. Though, now that I think about it, if I say that it doesn't run Windows, that might fend off some of the stupid questions. UMPC, like computer, carries connotations of an x86 processor that I'd like to avoid. I called my clamshell Zaurus a PDA with a laptop form factor.
I'd probably call it a high-end gaming PDA.