PLBs go to a 24 hour, 7 day a week monitoring system run by the U.S. Coast Guard. If you are in view of a satellite, the system will pick it up. If you're not, you're probably in an area so remote that even a portable amateur radio transmitter won't help you, unless you're using HF. Yes, there are limitations to the satellite system, just as there is with Amateur Radio.

http://en.wikipedia.org/wiki/Emergency_Position-Indicating_Radio_Beacon

It is also worth noting that after February 1, 2009, analog beacons of any type will not longer be monitored.

http://www.cospas-sarsat.org/FirstPage/121.5PhaseOut.htm