In updating an existing Rails app to run under Rails 3 (beta 4), I realized you could indicate that a certain test case should be skipped. Literally, just add “skip” inside of the test (I added it to the top of one of mine as it was actually a work in progress) followed by an optional message. For example:
test "should take the warp core offline" do skip warp_core.dismount end
Very useful.