I like a shoulder bag more than a day pack for commuting. The key to a good shoulder bag is the shoulder strap. Find one that is contoured and slip proof. A shoulder bag can be simply set between your feet when riding the T or carried in your hand while maneuvering tight aisles. A pack is too bulkie on the back and unyielding in tight spaces. When worn with only one strap over a shoulder you're forced to keep a pack in place with a hand.

If you really want a day pack, I'm sure you can find one that fits at one of the specialty backpacking stores (REI, EMS, etc). Many high end packs come with different sized shoulder straps.

Check out Patagonia's "One Bag". It's about 1300 cubic inches (the size of a big day pack), has a decent shoulder strap, a couple of handles, and converts to a back pack when necessary. It's their smaller version of the MLC carry on bag.

OneTim