If you can't afford McCartney's Stella(r) prices, there's a great little tutorial here on how to make a laptop bag from an old pillowcase.
OK, so perhaps the print that's been chosen for this bag isn't the coolest you've ever seen, but you can use any pillowcase you like, and if you're concerned about security while you're out and about, I guess the shabbier it looks, the less likely any potential muggers will are to know you're carrying a shiny bit of kit in there!
SAVE THE CAVES
i totally agree with that comment Jacinta
LOVE this - I have been making things from recycled materials for http://www.batteryfast.com years and wrote a book on it called EcoDeco. This ticks all the boxes and I can't wait to make one!