P914 writes –
When my Fastrak tag stopped working, I decided to risk losing my $20 deposit in the name of science. It turns out that they open pretty easily by prying around the edges with a flat blade screwdriver. I posted a few photos to Flickr so everyone can see what these look like inside. Maybe someone else knows more about RFID and can explain how the circuit works?
Fastrak autopsy (photos) – Link.