Wow- I thought I was the only one who felt this way! While Salma Hayek looked enough like Frida, (albeit, somewhat) and if a scene called for her being sad, happy or whatever, she was able to "act" sad, happy, etc-she just wasn't able to bring her to life.
I think there are any number of excellent but perhaps undiscovered actresses that could have played the role of Frida. Perhaps an actress who knows what the average person goes through in a place like Mexico, who has experienced real injustice, and/or what it is to really be working class....?

My guess is that the studios thought they had to get someone like Salma and other "A-List" celebrities in order for a film about Frida Kahlo to "sell". I think they underestimated just how greatly admired Frida is and how a good but unknown actress would have been just as equally sucessful. I did enjoy watching Alfred Molina as Diego though. I also thought I was the only one who believes Madonna can't really act, and uses her power in a manipulative-as opposed to empowered- way.

BTW, has anyone seen the movie "Frida" from Mexico? It was directed by Paul Leduc, made in 1988 and it starred Ofelia Medina. That movie had an interesting portrayal of Frida. Not a lot of dialogue, but more re-enactments of various scenes of her life.