Eric, would you like to work on this?

Today during the Austin Group meeting (the folks responsible for POSIX), it was decided that -mount and -xdev should have a subtle difference in behavior, in order to reconcile some differences between implementations and against some other file-traversal programs like pax (the POSIX replacement to tar). The difference stems on the question of whether, when requesting that a cross-device file system entry (whether a mount point, or whether a symlink that crosses mount points when -L follows symlinks), should be visited or omitted during the traversal.  Both styles have their merits.  Quoting from the discussion results:

http://austingroupbugs.net/view.php?id=1133
"
    When restricting the search to files on one file system, it can sometimes be desirable for the crossing points themselves to be acted on and sometimes for them not to be acted on. (Crossing points are mount points and, if the -L option is specified, symbolic links to directories on other file systems.) The -xdev primary acts on them and the -mount primary does not. However, -mount also does not act on symbolic links to non-directory files on other file systems (if -L is specified). If there is a need for an application to exclude crossing points but include symbolic links to non-directory files on other file systems, this can be achieved by using two find commands as follows:

        find -L dir -mount -type d -print
        find -L dir -xdev ! -type d -print

    (in a subshell whose output is piped to sort, if the order matters).

    If both -mount and -xdev are specified, find obeys both primaries but the end result is the same as if -xdev were not specified.
"

We'll need to patch GNU findutils to match, in order to make -mount skip crossing points (while keeping -xdev with its behavior of visiting crossing points).