It hasn't taken long for people to trip over this one and I knew I'd see this one coming. People are finding when trying to boot or install Solaris 11 on relatively old hardware, they are getting output similar to the following:

Rebooting with command: boot cdrom - nowin
Boot device: /pci@1e,600000/pci@0/pci@1/pci@0/ide@1f/cdrom@0,0:f  File and args: - nowin
Error: 'SUNW,UltraSPARC-IIIi' is not supported by this release of Solaris.
Program terminated
{1} ok

This is because Solaris 11 does NOT include support for any of the UltraSPARC I, II, IIe, III, IIIi, III+, IV and IV+ CPUs as documented:

End of Support for Legacy Hardware

Support for legacy systems that have included the UltraSPARC I, II, IIe, III, IIIi, III+, IV and IV+ processor architectures (as reported by the Solaris 'psrinfo -pv' command) has been removed. All Oracle SPARC Enterprise M-Series Servers and Oracle SPARC T-Series Servers will continue to be supported.

Note: Oracle Solaris 10 will continue to be a supported operating system for these affected platforms as per the current Oracle lifetime support policy.

Unfortunately, I'm not privy to the reasoning behind this decision, though I suspect it may have something to do with the end-of-life dates for the hardware involved.

Of course you can continue to run Solaris 10 on these machines as the above states. You can also run Solaris 11 on x86, but just not 32-bit CPUs as 32-bit kernel support has also been removed in Solaris 11. 32-bit applications will continue to run without change.