>>> 
>>> I know I have successfully set up LDAP *and* NFS, because I can log in
>>> remotely with SSH as well as on a terminal.  However, when I try to log
>> in
>>> with GDM or XDM, it will accept my password, black out the screen, and
>>> then return to either GDM or XDM.  I *can*, however, log in with a local
>>> account.

In my experience, if there's an auth failure, GDM prints a warning on the login window, rather than blacking out the screen, and dumping you back to a login window. Seeing as you can auth via other methods, I don't think this is the case. You can probably find some info as to what's happening in the /var/log/gdm/* log files.

To me, it sounds to me like there's a crash somewhere in your gnome session. Is there any relevant information in the ldap user's ~/.xsession-errors file?

I've also seen this happen if there's a corrupted session state file (~/.gnome2/session). You can try removing that, and see if if clears anything up for you.

-Steve