No I don't, but have thought about it.
Will work same as a laptop. If your taking it out on the road with you, then you have to consider that you have all your information, round, prices, customers details. Should it get stolen, then someone has every detail of your business.
Yes you can password protect it, but if someone really wants the information from the netbook, its easy to get.
You could go down the route of a dongle and update to the net as you go along.
For me I have decided to stick with the work sheets. That way should someone break into my van , then I have only lost 1 days information and it would be easy to contact customers.