A friend of mine has the Dell Mini 9 and is very happy with it for light 
Rails development. It's no longer available, having been replaced by the 
Mini 10v, but that's still cheap and light, and you can buy it without 
the Windows tax.

I have an old Thinkpad X30 which I'm very happy with, modulo some 
problems with battery charging. I'd be interested to hear how its 
processor (Pentium Mobile 1.2GHz) compares performance-wise with the 
current crop of Notebook processors (e.g. Atom 1.6GHz)
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