Yeah - Im gonna create a branch called mobile_theme. Where I will post all of my work here. This makes the most sense.
If the scope of the changes is large, a development branch sounds like a good idea. If it's relatively small, it might be better to post a patch file in JIRA. Either way, I agree that we probably don't want to worry about putting it in RC2… but if it's accessible either through SVN or JIRA, we at least have something we can start playing with in preparation for 1.0.