Yep, it's happening in the wild. In 10g, Oracle CRS is *required* but it can be used along with third party clusterware as well. The migration procedures are outlined and the DBUA in 10g will migrate a 9i RAC environment to a 10g RAC environment (a valuable new feature).
I'm the IOUG RAC SIG's event chairperson, so if you haven't heard of the RAC SIG, I'd encourage you to check it out. Go to http://www.oracleracsig.org/ and you'll see a forum area along with other interesting events, like RAC topic web seminars presented by Oracle and non-Oracle experts. If you're an IOUG member, you can also join the IOUG RAC SIG mailing list server.
RAC environments are complex, so it's especially important to do at least one test upgrade on a non-production environment to practice the procedures as well as test your application on the new release (to flush out any performance changes due to optimizer plan changes, etc.).
This was first published in September 2004