Depends on the server, even rebooting of 'half' a server is possible.
If server has 2 or more sockets (T5-2, T7-2 .. ) you can have two (or more) service domains.
Lets say there are 2 sockets, with 2 controllers, 2 network cards 2 hba etc.
Those two service domains (primary and secondary) will hold 'PCI endpoints' to each of those mentioned above (1 per domain).
Guest domains (LDOMs) will be on on primary or secondary domain.
You can then actually achieve zero downtime when patching hypervisor.
Live migrating ldoms from primary to secondary, doing patching, reboot just primary etc.
But i would not advise it, if you don't actually require it real bad.
It increases complexity and maintenance.
If you have multiple sparc boxes you can always live migrate to another box.
Just be sure you are in 'the matrix' regarding OS versions and FW levels.
Also, read the fine print for OVM server, especially known issues
Take a look at this article :
Availability Best Practices - Availability Using Multiple Service Domains (Virtually All The Time)
If you plan such configuration, plan carefully and read the exact docs for the version of Solaris you are using.
Hope that helps
Regards
Peasant.