406 Beacons also contain a 121.5 beacon for SAR asset homing. The 406 beacon is picked up by satellite, and a position passed to the ground station. The SAR helicopter etc will then conduct the final homing to the reported position using the 121.5 signal. To the best of my knowledge ( current up until earlier this year) there is no current plan to upgrade all SAR assets to home to 406 MHz.
The 406 beacon satellite coverage is almost worldwide so unless you are 'really' lost it should pick you up. The areas without coverage are shown on several websites but unless you are planning on a trip to some really remote parts of the Southern Ocean you should be fine.
The Wikipedia entry for '406 Beacon' will tell you more than you ever wanted to know, should you be interested.