I've had a couple of last gen 12" pb's and have hackintoshed an msi wind and lenovo s10. The msi had a mattel build quality that was offputting, and both had compromises in their keyboards (D@%n you, right shift key!) and in their postage stamp sized keyboards.
The 12" had a great form/size factor (if they made the same thing with a c2d and nvidia 9400, I'd be outside of the applestore before they opened today) and could install numbered os updates with impunity.
Newer model netbooks have improved keyboards, like the hp mini 1000 and dell mini 10v, but even these have compromises in their improved trackpads. HP has buttons on the sides of the pad, while the dell has buttons underneath them in a halfa$$ed rip off of the current mbp models. Your brother should lay hands on what he's looking at to make sure he can use it comfortably.
The choice comes down to what price your brother is wanting to pay, what he's wanting to do with it, and the level of hands on geekiness that he's able and or willing to do to install it and keep it running.
Does he want to do pure lightweight online mac stuff and not wanting to frak with the os? 12" pb is a winner, hands down, especially if he can get the seller to come down on price. If he's got a small windows program or two that he's wanting to use, and is willing to put in the time to get the machine to dual boot, a hacintosh mini (or nano, whatever) is the choice.
Not having enough distractions and aggravations in my life, I thought I'd hackintosh one of the newer 10" netbooks. After looking at the current crop, I choose the dell mini 10v due to its fanless operation, and its promise of tech support from the mydellmini folks for osx86. Like the machine, but even it's not perfect with its trackpad and anemic battery. (2 hours? Come on!) It's backup disks have a conventional windows install, so it promises to be easy to reinstall windows after the hd is partitioned.
To muddle the water further, I installed ubuntu 9 netbook remix on it as a lark, and I'm really impressed with it. So much so that I'm undecided on whether I want to do a dual or triple boot install or just leave it linux. It's secure, stable, comes with capable software already installed, and makes great use of the screen to choose and use applications. After a slight learning curve, I'd rank it right behind osx in ease and enjoyment of use.
Sorry about the thesis of a response.
