I believe that android switched from UMS to MTP to avoid problems with OTA updates while a drive was shared with a host PC.
You can change an android device from MTP to UMS by unloading the MTP gadget and loading a USB Mass Storage Device gadget in the android device. Or you can change your startup scripts to always start up as a UMS device. But remember that the default USB gadget was changed for a reason (as mentioned above).
You might find this useful:
http://forum.xda-developers.com/showthread.php?t=683223