OpenSolaris (the distribution) does not currently run on SPARC machines. One of the aims of the 2008.11 release is a SPARC version, but nothing has yet been released.
Your best bet is probably to try the latest build of Solaris Express Community Edition, which is based on opensolaris (the codebase) with the addition of various closed source parts, including the old installer which is used to do jumpstart installs. You can access the various distros here: http://www.opensolaris.org/os/downloads/ Cheers Andrew. This message posted from opensolaris.org
