See http://twitter.com/#!/hayafirst/status/28368786944

And my responses:
Check out custom exit behaviour: http://bit.ly/953iOR - could raise an exception after result change. See http://bit.ly/9PSaOt
Basically, cache the result.covered_percent somewhere, then later compare it with previous and cause an exit with status > 0

This could be an (optional) built-in functionality.
The SimpleCov.result.covered_percent should be stored in coverage_path/last_coverage_percent, then compared with the current run.

Configure with:

SimpleCov.start(:rails) do
  check_coverage_change 
end