Note: the Theme Check Plugin will also alert that a Theme is using TimThumb.

This is the Plugin used by the Theme Review Team, and TimThumb alerts as a warning-level notice, which means that a Theme with TimThumb bundled won't even pass the Theme Repository uploader script checks. Themes using TimThumb are no longer accepted in the official repository, so if you want to be certain that you are not vulnerable, I recommend using a Theme from the official repository.