The 406 ELT uses a new frequency for triangulation that is much more accurate. The old 121.5 systems use a system that is somewhat unreliable as they can get quite a few false readings as well as they can go off quite easily. The newer units use a satellite based system. If the older unit goes off, you are looking at a search radius of about 20 square kilometers. The 406 will give a search area of about 28 square kilometers. There is an optional NAV interface unit that will get SAR a search area of .031 square kilomters.
If the new ELT goes off, the SARSAT system will give your emergency contact a call within roughly 30 minutes. Have a customer that had a hard landing and set off his ELT. His wife called him within 15 minutes wondering what was going on as the Search and Rescue had already called her. They are a little pricey right now. You would be looking at about $949 without installation. Their may be a couple of companies coming out with something cheaper in the next year or so. Wouldn't guess that it will be a lot cheaper. Don't expect to see something priced like the Ameri-King 450 in other words.
The main thing, is the safety factor. If you go down with a 121.5 unit, you may not see help for several hours. With the new units, you may see rescue within an hour. If you are injured, that could mean the difference. You could fly with a Personal Locator Beacon. They use the same system as the installed ELTs. The only issue there, would be if you could activate it after a crash or during the descent. Also, they do not have a G switch.
As far as a mandate, the FAA has not given any indication that they will mandate the switch. Transport Canada has. Expect AOPA to fight any kind of a mandate at this point. However, the COPSAS/SARSAT system ( which monitors for ELT transmission) will stop monitoring the 121.5 freq on Feb 1st of 2009. So, it is more of an issue of when you want to do it.
