It looks like the crossover build hasn't caught up with the native
(imake) build in regards to IPv6. The native build sets -DIPv6 in
response to BuildIPv6=YES. I tried adding -DIPv6 to
${X11FLAGS.CONNECTION} in bsd.x11.mk, but it's apparently not quite
that simple...

The Xserver is listening on IPv6:

    # lsof -i :X11
    COMMAND PID USER   FD   TYPE     DEVICE SIZE/OFF NODE NAME
    XFree86 237 root    1u  IPv6 0xc18cd13c      0t0  TCP *:X11 (LISTEN)
    XFree86 237 root    3u  IPv4 0xc18cd278      0t0  TCP *:X11 (LISTEN)

but

    # DISPLAY=inet6/localhost:0 xdpyinfo
    Xlib: connection to "localhost:0.0" refused by server
    Xlib: Protocol not supported by server

    xdpyinfo:  unable to open display "inet6/localhost:0".

I seem to remember all that working with an out-of-the-box build of an
XFree86 4.4.0 release candidate.

There's another wrinkle: XDM-AUTHORIZATION-1 isn't supposed to work
without working IPv6-mapped IPv4 addresses, or at least that's how I
read the release notes. I set "net.inet6.ip6.v6only=0" and build xdm
with -DIPv6. It works (with inet/ only - see above). We were supposed
to discuss changing that by default, weren't we? For the meantime,
we'll probably want to track the native build policy by adding -UIPv6
to Makefile.xdm.

Frederick