When I took the wonderful Pragmatic Rails Studio in January, Dave Thomas described the
rake recent task that runs tests for all app files that have been modified in the last 10 minutes. I asked Dave if there was a rake task for running tests on all files that had been modified since last subversion update/commit. He said I should write one, so I did.
You can grab it from the goodies collection: test_modified.rake. Let me know if it has any issues.