This problem also occurs on my Indigo system, and removing the partitioner postinstall script *may* help continue, (and I get the same errors in /var/log/syslog on the running installer) but it does not work in the case where you are setting it up with a new disk.
Exiting to a shell and trying to run fdisk results in almost being able to construct a working SGI label here, except I cannot create more than one partition. I am going to attempt disk layout from a debian 6 image, and see if I can get it to continue with that.