Ah yes, you should execute an Affidavit of Loss for your plates. I think you should file it with the LTO. Have a police report filed na rin, so that you're *$$ is covered. Keep copies of these documents in the car, so in case you're flagged down, you have something to show the apprehending officers. The police will definitely go after you first when they apprehend the car used in some monkey business with your plates on it. In the absence of a police report and affidavit, they'll tag you as an accomplice at the least. I can't recall anymore how long it took for the replacement plates to arrive, so my dad had a temporary plastic plate made.

I remember a story on this thread, posted by Otep. His kumander's plate was stolen, so he had a really nice temporary plate made. Instead of "Philippines 2000" or what have you, written was "Lost Plate" or something to that effect. Lo and behold, pinag-initan pa ng pulis. Well, what happened next is another story altogether.