have to agree - either get a messenger bag with a computer sleeve or get a back pack. DO NOT get a regular shoulder bag unless you've got a rack to bungy it to. Messenger bags have a second strap(or third) to hold them in place on your back.

The best
http://www.pacdesigns.com/oversize.htm

the rest
http://www.chromebags.com/kremlin.php
http://www.timbuk2.com/tb2/catalog/c...?categoryId=45
http://www.manhattanportage.com/cata...products_id=89

Personally I'd go for the original back pack I recommended but if you're set on a shoulder/messenger bag then look at the links above.