From what I have found, it sounds like Linux currently can only mount the FreeBSD FFS in read only mode. I've been able to do that, but I'm just trying to make sure I'm not missing something to get write access.
Edit: I figured out mounting ext3 under FreeBSD. I poked around and found the kernel module to build. Why FreeBSD doesn't install that by default when you install 'linux compatibility' beats me. I kind of expected it would do so - 'mount_ext2fs' was there, but not the module it needed to work.