I'd say not very much. I've been through the tutorial in both 2.3.5 and 3.0.0.beta and think the improvements in the latter justify developing in it. Probably

I'd say not very much. I've been through the tutorial in both 2.3.5 and 3.0.0.beta and think the improvements in the latter justify developing in it. Probably a much more useful question to ask yourself is whether or not the plugins you'll need for your application have been tested, and work in Rails 3.