Well, even though this is pretty much entirely what you didn't ask for, I'd say for the price of a tablet go with something like the
ASUS Taichi or
Acer Aspire. If your handlebar bags are big enough for the 11.6" form factor you'll get a much more well-rounded device than any tablet out there, similar battery life (as in "it works all day then I charge it at night"), and still lightweight enough that you shouldn't even notice it's there.
The biggest reason to go with something like that (or a Transformer tablet) is the actual keyboard. You say you want to use this for writing. Nobody I know (which, admittedly, is a tiny, tiny selection of "all the people") would seriously consider doing more writing than a short email reply on a touchscreen virtual keyboard.
It's almost always true that the more your gadget can do, the more you grow to rely on those capabilities - see, eg, hundreds of millions of hysterical posts about who has the better App store, Apple or Android. Buy in on a laptop that's as portable as a tablet, and you will probably find that in a little while you're not so keen on surrendering your capabilities. USB ports, removable media, ability to plug up to display projectors, better integration with work software, and many more. If it's just a toy you want, there's nothing wrong with that, get an iPad or Transformer, but if you want to do more than just passively absorb media, tablets are not your best bet.
EDIT: If for whatever reason the weight of a small laptop is going to be a problem, then you should be looking at the 7-9" tablets, but you'll probably not find a good way to integrate a keyboard with something that size.