Rails 3.1.0.rc8 has been released (we've an issue with rc7). This is the final release candidate. Please give it a try, it's our last chance to fix regressions and severe issues. We will be releasing final 3.1.0 next August 30th.
Check the CHANGELOG file of each framework to see what we've changed.
You can also see issues we haven't closed.
A comprehensive CHANGELOG will be announced when 3.1.0 final is released.