Linux x86-64, RHEL5.8, upgrading 22.214.171.124 to 126.96.36.199JAN2013 CPU, 3rd of 7 databases, first two had no problems, using DBUA, ran successfully. noticed afterwards a lot of things breaking. Traced to /etc/oratab being reset to 0 bytes. In the first two cases DBUA had correctly updated OH to the new value but for this one it had done the wrong thing.
Just a warning - keep an oratab backup, you never know. Yes I know you can recreate it but a PITA and a delay.