Number of Mexicans in the USA who voted in the recent (Mexican) Presidental election: 28,000.
I find this to be remarkably low. 58 percent of those voted for Calderon, although presumably the small number of voters corresponds to a strong selection effect rather than a representative sample.
Here is one account, although it does not look into the reasons for non-voting very deeply. Thanks to Sergio Hernandez for the pointer.