I\'ve had problems with autotest before which I resolved by downgrading from ZenTest 4.1.4 to 4.1.3.
$ ruby -v ruby 1.8.7 (2008-08-11 patchlevel 72) [univers
I can also confirm that simply installing 'autotest-rails' solved the problem for me without having to downgrade ZenTest to 4.0.0