If you travel a lot with your laptop, may I humbly recommend this little backpack from Incase? I bought one this week after my existing laptop bag gouged a nice deep cut in my shoulder. It’s got a dedicated laptop sleeve big enough for a 17″ machine, enough room for all the crap you carry when you’re travelling, and it’s got secret pockets for stuff like passports, keys, iPods and things like that. Best fifty quid I’ve ever spent. My one came from the Apple Store but I’m assuming you can get them from the usual laptop-related shops too.
Plain old laptop bags seem like a great idea until you’ve lugged them around for 24 hours.