From the limited amount of research I have done, it looks like once south of Humphrey's Peak in AZ, San Jacinto in CA, and Sierra Blanca in NM, there do not appear to be any high mountains in Mexico until one gets to central Mexico and the Volcanoes around Mexico City. Is this the case?

Edited to add: high mountains, such as San Jacinto and others, being about 10,000' and over.

possibly

Here is a couple. Certainly not like the volcanoes around Mexico City. Nonetheless it looks like an interesting area.

http://www.summitpost.org/mountain/rock/150367/las-nieves.html

http://www.summitpost.org/mountain/rock/150366/la-viga.html

Baja has a nice range called Sierra de San Pedro Mártir http://www.oceanoasis.org/fieldguide/sanpedromartir.html its highest peak is El Picacho del Diablo at 3100 meters. Here is Bob Burd's trip report http://www.snwburd.com/bob/trip_reports/picacho_del_diablo_1.html

Depends how high is high. If high is greater than 20k, then south of Denali, there are no high mountains until South America.

I second Picacho del Diablo as a very worthy peak. It is a beautiful mountain isolated in a very rugged range. It is well worth a visit (or two). Totally unexpected to see pine forests and snow in baja. On my visit, we saw a mountain lion about 40 meters to the side of the road pacing my car. It followed us for about a minute before it got bored.

So not many it seems, but there are a few there.