Wayan at OLPCNews.com has an article up on what he's terming "3P computing" (Power, Price, Performance). Some of the commentators and myself would also add "Portability" to the list.
I think there's still a lot to shake out to give this market it's own identity -- something more than an iPhone or PDA, but still less than a full (desktop-replacement-style) laptop. The problem is that there is also a growing small(ish) but high-powered laptop market (the Air, Fujitsu's lifebook, and many of the portable tablet PCs), so the mini-note (HP's working term) or 3P (Wayan's) will have to compete on price as well as a set of features that the high-powered minis can't beat. This I think is where an expanded notion of Portability comes in.
Portability doesn't just mean small, and it doesn't just mean light. It also means ruggedness -- I need to toss it in a bag and go, not spend 10 minutes repacking it in a specially-designed single-use laptop bag. True portability also requires serious low power consumption (one of the three Ps); which high-powered laptops will really face problems with. Low power consumption means more battery life for the same battery weight (adding to the lightness), and it means not searching for an outlet at every layover, coffeeshop, porch, and so on.
I'd love for vendors to also start building in cell-network connectivity (GPRS/EDGE/EV-DO) with some bundled, reasonable, data plans -- continuing the path from hunting for an ethernet jack to the constant hunt for an open wifi signal to just being connected.