Oracle have started giving end-to-end recommendation for database software and hardware platforms. Recommending a server, storage system and operating system for new deployments. No need to ask around any more. Just go to this page
Oddly enough, Oracle chose Linux x86_64 for all their recommended configurations so far, perhaps the Solaris choices are to obvious to list. They claim they see a very high demand for Linux x86_64 systems at the moment. Not suprised by that, large SGA allocation on 32-bit platforms has always been a pain in the neck.
We've had a Sun v20z running 64-bit CentOS for six months or so and haven't seen any major problems. Did see one core dump issue in data guard when configuring the standby database in dgmgrl.
I'm sure we'll see more (Unix) recommended configurations in time.
And still no sight of Ubuntu.