Luggage rack attachments for laptops?
Your best bet for weatherproof and crash worthiness is going to be a pelican case. They have a whole series of cases for laptops.
http://www.pelican.ca/case_group_search.php?CaseGroup=Laptop

I've personally taken a 'hardback' case on a paddling trip and it held up great. Many pelican products also have a warranty against failure as well so if the product does leak you've at least got a new case and a replacement laptop.

If you get one of their other cases you can also lock the case which will assist in keeping it secure from a thief. Yeah it's not going to stop a determined person but at the least it will give you time to realize and change any vital personal information.

Past that make sure you backup your data with something like Mozy.com. At some point you'll have a drive failure no matter what.