Rspec 2 introduce a very efficient way to test only one test or one test suit, it's filter_run.
You should first add filter_run in rspec/spec_helper.rb
config.filter_run :focus => true
Then you can tell rspec to test only one test you are focused by
it "should focus now", :focus => true do
...
end
rspec will only test this spec, :focus => true can be applied on describe/context as well.
One problem is that if there is no :focus => true on your tests, rspec will do nothing, but most of time we are expecting to test all specs if no focus is true, so you should add a line to spec_helper as well.
config.run_all_when_everything_filtered = true
As the name implies, rspec will test all specs if no focus filter.
Another you may interest that you can also define filter_run_excluding
config.filter_run_excluding :slow => true
rspec will run all specs except what specs are marked as slow.