UPDATE, Dec 15 2008: As some of the fixes have been integrated back into OpenSolaris 2008.05, this is even simpler now.
Please see below at Evan's comment.
UPDATE, Dec 18 2008: The screencast now also reflects the simpler method.
To my surprise, I've come across a few blogs and comments about the new OpenSolaris 2008.11, that mentioned that it is not possible to upgrade from OpenSolaris 2008.05 to this latest version.
Although it's not as straightforward as it should be, and as it is, in fact, now (with OpenSolaris 2008.11), but it is still possible on command line, with a few simple steps.
Checkout the screencast on how to do this:
In addition, the detailed step-by-step instructions are also posted on opensolaris.org.
Happy Upgrading !