Apparently an Android debugging driver that breaks MTP was included in the August Windows patch cycle. You can fix it by:
- plugging in the device
- going to the device manager and finding the device. You should see Android Composite ADB interface.
- right-click on the ADB interface line. Choose "Update driver software"
- click "Browse my computer for driver software"
- Click "Let me pick ..."
- Choose the MTP driver.