Sure, I've been struggling with this on XenServer 5.0 and 5.5. Currently I have no success logging into the XenServer 5.5 setup, but I remember modifying the PAM modules on 5.0 resulting in a working solution. Unfortunately, I've forgotten what I did...

But I assume PAM is the best place to start to troubleshoot the issue. If you should be able to figure out how to mod the pam setup, please let us know.