im pretty much just going to repeat what others say on this subject. do your depth of field in post. i was in the same situation the other week and started with the mental ray DOF, it took 13 hours for 200 frames, on an athlon 1.2, yickes!! did the same thing using digital fusion and it took about 1 hour, which included keying in the f-curve...and it looked better.
what you need is so easy to achieve in a package like after effects or digital fusion that you really would be wasting your time with the mental ray DOF.
sorry this isn't actually answering you question, but im just trying to save you ALOT of time. if you need any specific details then please ask and i'll try and help, although im no expert!!
leach