Well, long story made short: I've ditched Solaris 10 x86_64 in favour of Red Hat Enterprise 5.1. Why? For one thing the Solaris 10 installation was frustratingly complicated.
Performing an installation via serial console requires you to redirect output from the BNC to the System in the BIOS settings. Then the manual states you should choose grub menu's option "ttb". Of course the manual meant "ttyb". But it is actually a typo AND an error: you must choose ttya or all you'll see after boot is "Sun Solaris 5.10" and nothing else.
Once the serial console installation is finished and the system reboots you have another obstacle. The boot process stops at this error message:
Bad PBR sigGoogling around (Sun's KB was useless) I found someone with the same problem reporting that a GUI installation didn't show that error. Very well, I configured the firewall to allow Sun's network KVM:
Reboot and Select proper Boot device or
Insert Boot Media in selected Boot device and press a key
8890 TCP Remote Console
9000 TCP Remote Console
9001 TCP Remote Console
9002 TCP Remote Console
9003 TCP Remote Console
69 UDP TFTP (for firmware upgrades)
161 UDP SNMP (for monitoring)
As the anonymous poster reported, this did solve the problem. Now to install postgres for x86_64. Hmm. Doesn't run - complains about missing libraries. Probably just need to run crle to set-up the location of the 64bit libraries. Where are they, where are they... WTF? There are no x86_64 libraries installed in my Solaris 10 for x86_64. I check around with `file' and yeah: all bloody system libraries are 32 bits.
I confess I didn't try too hard after this. A day later and we had postgres running on RHEL 5.1. And yes, with a 64bit binary and libraries.
To Sun's credit, the hardware looks a beauty, even though you have to assemble the whole thing by yourself: memory cards, SAS card, Fibre Channel card, hard disks and whatever extras you bought. What a pain in the arse. Compared to the other Dell servers with pre-installed RHEL 5.1 we recently bought the Sun experience is pathetic: the Dells where up on the same day versus 1 week for Sun.