I don't think netbooks would be fun to play tf2 on...
You won't be able to find any UMPC's that are able to run TF2 properly. Most UMPC's have either an Intel GMA-500 or 900 integrated GPU. Then you get into the Netbook world, but that too is not looking good. TF2 has been said to be more CPU intensive than other Source-based games, and as such won't run well on an Intel Atom processor.
Your best bet would be to look into an Asus N10 - although it's a little expensive for a netbook (when looking at it with the dedicated video card). Sadly there is no "small" device that can seem to play TF2 that well yet
Lifebook U1010
Intel A110 800MHz
Intel 945GU+ICH7U
1024MB DDR2 400
ATA-100,1.8' 4200 40GB
5.6 inch WSVGA 1024