Yes, the silent/cool(er than my other laptop) aspect played a major role in me buying the mini 10- I just wish I'd done some proper research before buying.
Are you not going to be using pb with an external midi keyboard? If you are then the chances are good that your keyboard will have a midi input too. This is the prefered playback method as it provides the lowest latency and is much nicer on your CPU. Any intel cpu from the last 5 or so years is going to have no probs in doing this, hence the only real concern is that you have a semi-decent graphics card with fully working drivers so you can achieve the opengl acceleration required for smooth scrolling. Practically every other intel graphics chipset released over the same period will have no probs doing this and are very well supported by all versions of Linux and other free nixs.
As you say, there really isn't that much to separate the different netbooks, thanks largely to intels continued near monopoly of the x86 market, which obviously they're getting fined heavily for as we speak. Thankfully, ARM netbooks are right round the corner and will likely totally out-do the atom in terms of energy efficiency, but you will of course lose the ability to run wine/virtualbox and any chance of running real Windows. I would imagine future ARM chips would eventually be fast enough to let you run virtualbox though?
I found this comparison of most netbooks currently available
http://en.wikipedia.org/wiki/Comparison_of_netbooksAs it stands, if you're hoping to run Linux then you want to avoid any netbook that uses the intel gma 500 or VIA (C7 /VX800) graphics or specifically states that it has no 3D acceleration. Maybe in time these chipsets will be better supported under Linux, I sure hope so!