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 experienced the same problem. Installing the autotest-rails gem solved it for me. I didn't find it necessary to downgrade ZenTest to 4.0.0 (running 4.1.4).