Speaking as someone who lived an hour from the Florida Keys for many years, Mexican limes are NOT the same as Key Limes!!

They're about the same size, but Mexican limes are sweeter and bright green on the outside. Key Limes are much more flavorful and tart, and the skin is a pale yellow-ish green. Mexican limes would make a very different pie than Key Limes - you wouldn't get that signature tartness.

Not to take this thread on a lime rant, but they ARE the same species, although there may be slight differences due to terrior. The skin color is an indication of ripeness (yellow actually means more ripe and less tart). FWIW, in many cases the fresh "key limes" you will encounter in the grocery store will be labeled as "product of Mexico."

http://www.keylimeshop.com/html/about_key_limes.html
http://www.hort.purdue.edu/newcrop/morton/mexican_lime.html

Whoops - my bad. I was mentally thinking Persian limes, which ARE different than Key/Mexican limes.

wikipedia