I had this exact same problem not too long ago and found that if you pull the entire AVCHD metadata folder structure with the .mts file, you should be able to utilize the log and transfer option in FCP7 to convert them to something FCP can handle. I, however, was not given the folder structure and ended up having to throw the whole project in Premiere, learning the program as I went, to make deadline. It handled the .mts files just fine... and turns out, Premiere is an extremely snazzy program.
Lisa Talley
Sacramento Video Production