I’ve got a story in the current issue of Science about the challenge of predicting how many species (and which) may become extinct due to global warming. You can read the article here on my web site. I blogged about some of the early material in the article back in 2004 here. For a good summary of the qualms many scientists have about the power of current models, check out this recent review in the journal Bioscience: pdf.
[Update: If for some reason you have trouble reading my article on my web site, the link to the story at Science is here.]