Sounds like PAM might be rejecting your input, I'm no pam expert though.
I have two suggestions.
First, to rule GDM out as an issue you can attempt to log into a regular console. Press the contrl+alt+f3 keys (Almost any f-key really will work) and you should be at a black screen prompt. Try and log in this way. Just press the alt+right/left arrow keys together a few times and you'll eventually be back at the GDM screen. (Usually it's ctrl+alt+f7)
Second suggestion, on the machine you are attempting to log into I recommend you watch /var/log/auth.log (as a user who can log in) and see what the output is when you try to log in using gdm.
tail -f /var/log/auth.log
Then make your GDM log in attempt with a long user name.
We should learn something after all this.
If you have admin access to the OpenLDAP server you can change the log level to a very verbost level and then follow the logs there too to see exactly what user name is attempting to authenticate when the failure happens.