How long do we have before Google lowers rankings on outdated CMS's (or boosts up to date sites), now that's a question I'd love Matt Cutts to answer.
There are some things I don't like about 3.0 from an SEO standpoint, duplicate content issues (multiple new url's and locations for same content) and it handles themes differently but my existing theme is sufficient already.
from the article...
Normally this is where I’d say we’re about to start work on 3.1, but we’re actually not. We’re going to take a release cycle off to focus on all of the things around WordPress. The growth of the community has been breathtaking, including over 10.3 million downloads of version 2.9, but so much of our effort has been focused on the core software it hasn’t left much time for anything else. Over the next three months we’re going to split into ninja/pirate teams focused on different areas of the around-WordPress experience, including the showcase, Codex, forums, profiles, update and compatibility APIs, theme directory, plugin directory, mailing lists, core plugins, wordcamp.org… the possibilities are endless.
emphasis mine, I'd like to take at least those three months to upgrade, until they are done being ninjas and I can figure out what needs changing for my particular site. This isn't just patching of the old system, it's a re-write in many ways.