If you're looking for something just to sit on your lap at home, get a low spec laptop.
If you want to cart it around in your bag, get a netbook.
Personally, I would highly recommend a good netbook. I have an Advent 4211 (a re-badged MSI Wind), and recently I purchased a 7800mAh battery for it for £50 online on top of what I paid for it (nothing, as I got it on a Vodafone mobile broadband contract). Takes the battery life up to 8hrs. Bloody perfect.
The other thing too is whether or not you want mobile broadband. Getting one on a mobile broadband contract can be a very good deal if you're going to pay for mobile broadband anyway!
(Posting from my Advent 4211 in O'Neills in Soho... all very mobile

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