There is huge consternation about fitting 406 ELT's on US registered aircraft in the USA.
Yes, the FAA requires an ELT, and has for a very long time.
Most ELT's now fitted broadcast on 121.5 and 243 Mhz, and the FAA have clearly stated that a mandatory switchover to 406 ELT's will not be required for ops in the USA.
However, flying to Canada and Mexico are a different story.
The problem arises when one desires to fit a 406 ELT to replace an older model.
Under the TSO granted to the new 406 ELT manufacturers, a greatly beefed-up mounting plate is required, because the new ELT's mounting must withstand 100G's.
Oftentimes, a DER must be hired to design/approve the structural modifications necessary, for this 100G requirement to be met.
This ain't cheap.
So, the portable units are selling like hotcakes at a Sunday breakfast airport fly-in.