if you telnet to a fresh RH 6.1 system as regular user, then you ask to 'su', you have a 'segmentation fault' after providing the password. bash and zsh tested. If you ask to 'su' after having 'rlogin'-ed, everything works. pam-0.68-10.i386.pam and usermode-1.18-1.i386.rpm don't fix the problem. More configuration details avaiable upon specific e-mail requests.
Cannot reproduce here, and I can't think of any situation that this would occur in except: 1. System has run out of memory or other resources (doesn't sound like this is the case here). 2. Incorrect PAM configuration for su. Since it works with the default configuration, this is the only thing I can think of.