When I shut off USB Debugging in the developer settings I get a USB Mass Storage prompt upon connecting the device to my computer. However, for the life of me I can't find a place to shut off this prompt (i.e., turn it into a notification rather than have the window pop up) or change to MTP mode (I'm assuming I no longer get Kies mode — I have a Vibrant). Does this exist?
If you go to Settings -> Storage, then hit the menu button, you'll find an item called USB computer connection.
Here you can set USB settings.