Look at Waterfield Designs (sfbags.com). They make laptop sleeves to fit most netbooks and also make a "piggyback" that clips on for other things (charger, etc). With this setup:

1) it's easy to put everything in a larger bag when you're traveling
2) it's a tidy package for general use (sleeve + piggyback)
3) use the strap with the piggyback solo for a shoulder bag

If you want to put the sleeve in something, their VertiGo might do the trick as a vertical bag.