I use the Brenthaven Duo.
http://www.brenthaven.com/catalog-duo-oversize.html
Both my Thinkpad T42 and my Dell Latitude D820 fit into the center section, as long as you don't overload the rest of the bag.
A massive shoulder strap, although I tend not to use it much because the bag also includes a slipover strap for the handle of my rollaboard.
I've had mine for about a year with no wear problems.
A minor quibble: When carrying only one laptop, it can sometimes be a bit unbalanced when setting on a desktop - it will want to flop over on its side sometimes.