The Following was posted on the Baja Bush Pilot's website

406MHz ELT verses EPIRB ELT in Mexico - 7/23/2008
 

That in order for an aircraft to fly in Mexican Airspace after July 1, 2008, the aircraft must have a 406MHz ELT installed in the aircraft.

However, for aircraft that do not have a 406MHz ELT, Mexico is making an allowance for continued use of existing 121.5 ELTs to July 1, 2009 or, when the battery on the existing 121.5 ELT is due to be replaced, which ever comes first.

(new) This 406MHz ELT must be hardwired in the aircraft with its associated activation switch(s).  The only exception is that a personal EPIRB may be substituted for the hardwired 406MHz ELT but the flight is restricted to “over-land” use only.

So, you can use an EPIRB to fly up and down Baja as well as fly on Mainland Mexico.  However, as an example, if you want to cross over the Sea of Cortez from Loreto to Guaymas, you must have a hardwired 406 ELT mounted in the aircraft.  The other choice is to go around the north end of Sea of Cortez.

There are no exceptions for VFR, IFR, types of aircraft, etc.  So, both a Super Cub and / or Citation flying at any elevation would have to have a hard wired 406 or stay over land the entire flight.