The sub-4lb 12" notebooks has been definitively a subnotebook since the standard sizes went up to 13-14" back around 1999-2000 (and the 12" ones I had before that, ie Toshiba Satellite 400 were larger-bodied and around 6-7lbs, just with a smaller screen)

2.6 lbs (E4200) or 3.2lbs (D430) is as light as some 10" netbooks, or around half the weight of a serious 14.1" or 15.4" notebook. And "smaller" is only valuable up to the point where the keyboard and screen don't start sacrificing utility. Some people can use 9" netbooks comfortably; I can't - with the exception of the "as wide as the 10" models" with a wider screen bezel, they're uncomfortable to type even briefly.

I've had an older (non-widescreen) 12" subnotebook in the past (sadly my then-employer's), and found it fully usable... and it would open in then-current (MRTC) coach legroom when a 14" non-widescreen would not. 14" widescreens were a little better, but coach clearances have gotten worse and "just under 5 pounds" is still a lot heavier than 2-3.

As for processing power, the gap between a single-core, non-out-of-order Atom and any current dual core processor is a lot larger than that between a 1.3-1.4ghz ULV Core 2 and a higher-speed non-ULV Core 2 - the only exception being the current mobile quad cores -- if you can actually find one.