I'm not really sure what I'm doing, but I have tried many permutations of the x11 settings that people recommend setting.

my linode is a fresh debian 6 install +
apt-get install eclipse
apt-get install xorg
apt-get install xauth
apt-get install x11-apps

$> echo $DISPLAY
localhost:10.0

$> less /etc/ssh/sshd_config

...

X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes
PrintMotd no
PrintLastLog yes
TCPKeepAlive yes

...

Client side, I have installed xming, on mouseover it reads

Xming Server:0.0

and my putty settings are default except

Connection>SSH>X11:

enable x11 forwarding: checked
X display location: 0:0
-------------------------------------------------------

$> xeyes
Error: Can't open display: localhost:10.0
$>

I sort of suspect the problem is in not initializing something something xauth. But I'm a total noob.

help is (obviously) appreciated greatly!

Are you ssh'ing directly to the user? Not through su? Otherwise you'll get authentication errors.

Try a ssh -vvv and then run xeyes to see some more verbose information.

I posted this concurrently on a lot of forums, and it is now solved.

I am pretty sure the problem lay in Xming fonts, and the problem was that I did not have Xming fonts.

It is stupid that this is a separate install. but it did fix my problem.