Carrying your laptop
I carry my laptop around with me, it’s a 17in so a bit heavy I found the courier back option started to give me back ache because the way the bag sits on your back makes it naturally inclined to twist when you put something heavy in it.
I’m with purplecup though, I wouldn’t carry my computer in a pannier or rack top bag, simply because when I’m carrying it it gets the benefit of inteligent (ish) suspensions.
My favoured option is to carry mine in a 30l rucksack with a padded back I’m gonna get a padded sleeve to put it in too at some point…
The Crumpler one here looks okay
http://www.google.co.uk/products?q=laptop+sleeve&btnG=Search+Products&hl=en