When I got my personalized plate for my bronco I put the front one inside on my windshield. I got pulled over for it. I didn't get ticketed though, just a warning.
Here's a link to the MT license plate law:
http://data.opi.mt.gov/bills/mca/61/3/61-3-301.htm
Section 4(a) specifies that they have to be on the bumper.
My brother has gotten crap for not having a license plate in the front, and another time fo having the dealership (Ressler) plate in the front instead of his regular plate (which he had in the back).
must have one license plate displayed on the front and one license plate displayed on the rear of the motor vehicle.

"When you want to be a somebody; you drive a new car...When you know who you are; you drive an old car."