Eagle Creek, Solo, Timbuk2, Briggs and Reily, Manhatten Portage, and a number of other brands all have "convertible" laptop backpacks. They're adaquate as both a backpack and a briefcase, but don't really shine as one or the other. Business oriented backpacks, like Samsonite, have become fairly common.

For laptop security, every laptop and desktop I have ever been issued came with a cable lock, automatic backup, and hard drive encryption. One particularly security concious (paranoid) employer issued cable alarms. If you made it 20 feet before all he11 broke loose, you were doing better then most.