The latest update for FCP, 6.0.2 includes support for AVCHD files. From the
release notes:
Final Cut Pro 6.0.2 includes support for AVCHD Full HD formats (full width, 1920 x 1080 pixels, interlaced) recorded by camcorders such as the Panasonic HDC-SD3 and HDC-SD5 camcorders. Also, spanned AVCHD clips are recognized.
Important: Final Cut Pro 6.0.1 did not recognize AVCHD spanned clips, so each segment of a spanned clip was ingested independently. Because Final Cut Pro 6.0.2 now recognizes spanned AVCHD clips, it may be impossible to reingest certain segments of a spanned AVCHD clip that you originally ingested in Final Cut Pro 6.0.1. In this case, you may need to use the Log and Transfer window to reingest the entire spanned clip and delete the outdated clips that represent spanned clip segments.