can't open display

[phurd]

Hey All,

Up until unpgrading to FC3 I would ssh from machinea on machineb and then run 
programs on machineb remotely from machinea.  I get confused by X 
client/server language.  Since upgrading machinea from FC2 to FC3 I get errors 
like:
: Gtk-WARNING **: cannot open display:
 Xt error: Can't open display:

So machinea is somehow refusing connections.  No errors are appearing in 
/var/log/messages.  I've tried turning off the firewall on machinea, and I've 
turned off the (well hidden) disallow TCP connections to X server option under 
Login Screen Setup, and turned on XDMCP (I donno why, it just felt good).

None of this has helped.

I could use a hint.

-P.