I am using 3.01-08 for Ubuntu as client. This is the one that downloads by default from the debian library. I have also tried it with 3.01-04 for Windows and for Mac, but same results. I will try 3.03-13 and see if there are any differences, but I doubt it.

Ok, you have really old clients. I have successful login with 3.01-13 in my Windows 7
I took it from here: http://x2go.obviously-nice.de/deb/pool-untested/x2goclient/x2goclient-3.01-13-setup.exe
I also have Ubuntu 10.10 as client, located on VMWare image. There I successfully connected and worked (had no sound, but this is something further). The client version is 3.01-14 (latest). I took it from here: http://x2go.obviously-nice.de/deb/pool-heuler/x2goclient/x2goclient_3.01-14_i386.deb

From your tests, it is clear that you have client version problem, and nothing with authentication.

>4. If authentication is good, but there is blank/black screen, then try
>another: OS, Client, Client version. As I said above, the latest version
>doesn't work in my scenario. Hint: Don't try to connect with client from the
>same server, as it is not possible. If you use Windows, try to find other
>OS, install other (virtual if possible) Windows, Linux, whatever is easy for
>you.

Again, I have tried it with clients on Linux, Mac, Windows, all the same result: full GDM desktop with audio for local accounts, blank screen (but all successful messages in the log) for LW/LDAP users.

Here is the moment to ask which version is your x2goserver. I'm using 3.0.1-5 for x2goserver; 3.0.1-1 for x2goserver-home; 3.0.1-1 for x2goserver-one. I've installed it through the repository added this line to /etc/apt/sources.list (then apt-get update):
deb http://x2go.obviously-nice.de/deb/ lenny main.

I guess (still didn't check) there is newer server version, but I used the official way through the repository. I guess this is stable one?

Bottom line: please, try to setup your x2go server and client as my versions.

************************

BTW, as I'm fearing that if/when I get this issue resolved with the black screen showing up, that I will then have the audio problems, I will add these comments:
-I have Likewise Open setup to connect to our domain as the Default Domain, thus all of our uid's do not use the format DOMAIN\user. Before, we all used these IDs, but since we currently only have one domain, I have set LW to use the default domain and so now we can login with just "user". This also affects our directories and thus my .x2go sessions look like:
/home/local/DOMAIN/nwhunt/.x2go/C-nwhunt-51-1292422868_stDGNOME_dp32 rather than /home/local/DOMAIN/nwhunt/.x2go/C-DOMAIN\nwhunt-51-1292422868_stDGNOME_dp32
And within that directory are indeed the pulse audio files you mentioned were NOT there when you included the ~/.x2go/C-DOMAIN\userid-....
.pulse-client.conf
.pulse-cookie

I will report back if I find anything more.