Yes. As of April 2010 you need to have a 406 ELT on board to fly into Mexico. Canada is in transition to requiring 406 ELTs by 2011, and since Feb 1, 2009 the international COSPA-SARSAT satellite system has discontinued satellite-based monitoring of the 121.5/243 MHz frequencies.
While there is no requirement in the United States to replace the first and second generation 121.5 MHz ELTs, after Feb 1, 2009, 121.5 /243 MHz distress signals transmitted from ELTs operating on the lower frequency will only be detected by ground-based receivers such as local airport facilities and air traffic control facilities or by over flying aircraft.
Although the existing 121.5-MHz ELT will still be “FAA Legal”, it offers limited locating assistance to a downed aircraft., and especially so for remote areas.
When ELTs were mandated in 1973, most GA aircraft were equipped with an ELT that transmits on the 121.5 MHz frequency, the designated international distress frequency. The original ELTs were manufactured to the specifications of an FAA technical standard order (TSO-C91) and have had an activation rate of less than 25 percent in actual crashes and a 97 percent false-alarm rate.
A more advanced model of ELT has been developed — the C126 ELT (406 MHz). This newest model activates 81-83 percent of the time and transmits a more accurate and near-instantaneous emergency signal by utilizing digital technology. This digital 406-MHz ELT also allows search and rescue personnel to have vital information specific to you and your aircraft.
Here's some more ELT info from AEA http://www.aea.net/governmentaffairs/regulatoryupdates_item.asp?ID=43