Rails 3 is currently in beta, and I’ve had a look through the changes to see what impact, if any they will have on SEO. One change is that ajax helpers now work unobtrusively which will help to clean up the code, resulting in smaller page sizes. This probably won’t have any effect, but it is an improvement.
The other thing is Rails 3 will be faster. Faster render times means that the google bot can index your site quicker and more often. The google bot detects load on the web server by its page render time. I wonder if it things RoR sites are always overloaded?!