As a close follower of both MLS and FMF, I've had a certain curiosity about how long it'll be until Mexican sides start shopping in the United States when the transfer season opens. Mexico certainly has the purchasing power, as FMF is the highest paid non-European league.
While checking the rumor sheets on a number of sites, forums, and other places of questionable credibility but often very accurate in predictions, I ran into a few surprises. Listed as "rumored" transfers were the following players:
Atlas - Eddie Johnson
Puebla - Carlos Pavon
Puebla - Luciano Emilio
Tigres - Ante Razov
According to the questionable sources listed above, these are all current MLS players who are catching attention of certain Mexican sides. The past season there were some very mild internet rumors [probably originated in the 3 meetings between the two sides] regarding Brian Ching, Dwayne de Rosario and CF Pachuca. The culminating point for these rumors was a mention on national television [TV Azteca] during a CF Pachuca game, where the commentators stated that these two players could very easily earn their living playing for any Mexican outfit.
American players have participated for Mexican sides before. Wynalda and Balboa played for Club Leon before MLS resurfaced, and in '95 Dominic Kinnear played for the "team of the decade" Necaxa. However, since MLS began, I don't think there has been a single American player [not of dual citizenship] playing for a First Division Mexican time.
With all these rumors starting to float, and greater exposure to the American game [both by virtue of better TV coverage of MLS, and through CONCACAF tournaments] it seems like just a matter of time before you start seeing certain MLS players, and specifically American players suiting up for Club America, CF Pachuca, Tigres UANL or other Mexican sides.