X11Forwarding requirements besides -x and X11Forwarding?

[Britton Leo Kerin <fsblk@aurora.uaf.edu>]

I have two hosts on my local network.  I can ssh -X from one to the other a
and X11Forwarding seems to work fine, setting display to localhost:10.0 as
expected.  In the other DISPLAY ends up empty.  Forwarding is enabled in
sshd_config on both machines.  Both can run xauth with about the same output.
ssh -v -v -v -X shows the same things on both (neither mentions the DISPLAY
environment variable at all).  I'm at a loss.  Is there some other X or 
ssh related option I've missed?

Thanks,
Britton