For serious protection from a laptop sleeve, I'd second the Tom Bihn line. Many of their models keep the laptop suspended an inch or so off the bottom, which sure does help in the event of a drop.

If you're carrying enough laptop related stuff to warrant a dedicated bag, I've had great success using Knog's Franks Dog and Ortlieb's Office Bag, both of which are messenger style bags that mount like panniers. I'd recommend the Ortlieb for weather protection, and the Knog for off-the-bike carrying comfort and style.