Another option to consider, if you have the room, is the Xootr Crossrack and Crossrack bag. Here's a pic of mine on my utility bike:



The bag has a shoulder strap and lifts right off the rack, so it can be carried like a book bag. It's very water resistant, able to take a heavy downpour for awhile. The rack mount itself has a quick release option for part of it, so the bulkiest part can be removed if you don't need it, leaving just the bolted on seatpost clamp part. You could replace the quick release pin with a nut and bolt for security if you're worried about someone pinching the rack while it's locked up.

The Crossrack bag is big, too, so you can fit lots of stuff. You can use the rack with any pannier (or even just hang a smaller backpack on there with some futzing), but the whole getup is pretty reasonable at about $80 for both rack and bag.