grunt.registerAsyncTask('async', 'lint compass');
grunt.registerTask('build', 'async test');
In the case above lint and compass would be executed at the same time using child processes and than test would be executed after both are run.
I have a similar idea but for running the items in a multitask in parallel. These are complementary IMO.
Closing this for now, but adding it into the notes for 0.5.