Well they're not using FDMA, they're actually using TDMA, and yes, PG county made a big switch recently to using pre-phase 2 P25, which is completely unmonitorable by anything out of the box. There's an extensive thread about this very subject in the Maryland forum.
I heard a rumor that PG fire won't go phase 2, they'll stay with a standard Phase 1 P25 system, which can be monitored, but that's very unconfirmed
We already have very brief articles on both FDMA and TDMA in the glossary section of the wiki. You're more than welcome to add to them
73 Mike