I just replaced the front pads on my S40 at 4150 miles. The inside pad, the one that is stationary, was worn considerably more than the pad backed by the piston. What causes this? Is there an adjustment I can make to make sure that doesn't happen to this set?

I noticed that the bike rolled a little easier after the change. From that I'm assuming the inside pad has been rubbing a little since I got the bike. That would jibe with the greater pad wear.

Yep, clean and lube.   Check Auto Zone for the lube.

Notes on things that need lubing in a dusty enviroments:
-  grease & oil, WD 40, CRC,... will pick up the dust and gum up or if the dust is abrasive, it will stop acting as a lube and.....      
- Graphite is a type of mineral, and even though it is considered a dry lube, it is slightly abrasive in tight fits.

I would use a bit of anti sieze on the pins that allow the calliper to bring each pad into the rotor equally. That way, it can actually put even forces on the pads & release them, too. I am soon to be doing this myself. Glad you mentioned this, so I wouldnt just swap out the pads & go.