Seeing about 25% speedup here with just the Rails 3.2 RC1.
Combined with rspec-rails 2.8.0.rc2 (which includes some Rails 3.2-related changes), I saw my test suite go from ~2.5s (Rails 3.2.0.rc1 w/ rspec 2.7.x) to ~0.43s (Rails 3.2.0.rc1 w/ rspec 2.8.0.rc2).
Combined with rspec-rails 2.8.0.rc2 (which includes some Rails 3.2-related changes), I saw my test suite go from ~2.5s (Rails 3.2.0.rc1 w/ rspec 2.7.x) to ~0.43s (Rails 3.2.0.rc1 w/ rspec 2.8.0.rc2).
Ruby 1.9.3p0 across the board.