Yesterday a new zone brand was integrated into Open Solaris: Solaris 10 Zones. It allows running Solaris 10 in a zone on Open Solaris. The technology will be very useful in some instances for enterprise customers once Solaris 11 is out or when deploying Open Solaris. This is very useful if you want to make use of latest hardware platforms and/or technologies delivered in Open Solaris (and Solaris 11 in the future) and you need to migrate your older environments running Solaris 8, 9 or 10 without introducing changes to them (to mitigate a risk or due to lack of resources to do a more detailed validation of a new environment). This is especially true with Open Solaris and Solaris 11 as they are so much different than older Solaris releases. Branded zones are a very neat solution to provide backward compatibility without putting too much hurdle on innovation or having most of the users who do not need it to pay the price of it.
See Jerry's blog entry about Solaris 10 Zones.