Two pitfalls: 1) You should add some text to indicate to people using screen readers that this field should remain blank. 2) You should make sure to name the field something unlikely to be autofilled. There are browser toolbars out there that will happily autofill even hidden fields and trigger your spam filter.

But other than that, I agree that this is extremely effective against the common sort of drive-by spam bot. (And, obviously, completely useless against any sort of advanced or targeted attack.)