finally I succeeded - OpenSolaris 2008.05 update to snv_91
It was a little bit a pain: updating always timed out, even if ‘PKG_CLIENT_TIMEOUT’ was set to a much higher value than the default. The update server ‘pkg.opensolaris.org’ did not seem to be very reliable - hopefully it is not running opensolaris itself.
However finally I succeeded - and I am now on snv_91:

More on OpenSolaris soon.