I exported a .csv and attempted to import it into the Mac Address Book application. I got the error "The selected file does not appear to be a valid comma separated values (csv) file or a valid tab delimited file. Choose a different file."
It took me a few tries of doing find-and-replace on the .csv, but eventually I got it to work by removing stray linebreaks that were in the file. These linebreaks were originally from a textarea in my form in which some people entered linebreaks.
It might be a good idea to remove the linebreaks from the .csv export.