I can confirm Jukey and N900@900's problems.

Jukey - as a workaround, you can go into the terminal and

root
killall fmtxd
fmtxd -d

Afterwords, things seem to work OK (i.e., you can start/stop the transmitter on region 4 frequencies). However, this is probably not the best solution if you're trying to get you're non-technically inclined friend in the passenger seat to turn on/off the FM Transmitter.

Any suggestions for getting the default power to 120 would be great!