Today the Mexican Peso is trading at $12.77 MXN = $1 USD. This has been the worst month since February 2010 for the peso.
The events that have affected the peso's value this week include:
Weaker than expected US economic data (as they say, when the US gets a cold, Mexico gets pneumonia!)
The report that China is reviewing its Euro bond holdings
The European debt crisis
In a related note, Hacienda (Mexico's IRS) has placed a restriction on dollar cash deposits in Mexico. This has created a parallel black market for dollars, which can pose a huge threat to the economy. The restriction was put in place in an effort to curb money laundering.