Assuming I am willing to fork over a lot more money and use much less powerful hardware running a glorified phone OS, all for the sake of having a smaller computer.
Already we have tablets running x86 with good enough battery life, better power than the ARM counterparts, and a full OS, with external hardware support.
Sure, it's not completely there yet, but considering it's $399 today for something that is pretty damned close (and close enough that my girlfriend just ordered one, to use for most of her primary computing, but coupled with the active digitizer for sketching; basically, a netbook), it's only a matter of time before this is completely doable!
Eventually low-power portable-class computing hardware will be as powerful as typical desktop systems today. Though there will likely always be a niche for even higher performance machines.
Price will come down with volume and greater competition as the market matures, as it always does.
And the OSes will get better as people spend more time on these devices and developers spend more time targetting them and working out the appropriate UI optimizations for mobile vs. desktop modes.
