Grecco Salem posts a tutorial for creating a quick and dirty key-able Depth Of Field in Maya 2012 by using the distance node to control the camera’s DOF value.
Grecco Writes: Just wanted to show everyone a very quick and easy way to get a key-able depth of field (DOF) pass in Maya 2012. If you wish to key the OutMax attribute (The “End” of the DOF) simply key the corresponding locator that the distance tool created. Remember to Parent constrain the 1st locator to the camera so that it follows your camera. You can also connect the OutMin to a separate distance tool if you wish using the same technique.