The jQuery Validation plugin works by examining the CSS classes assigned to an element, and will then apply preset validations (it's really awesome). I specifically wanted to validate an email form component, and the only way I could get it to work was to use jQuery to add the 'email' class to the element once the page had loaded.
Attached is a patch (webform_css_class_patch.diff) which implements the desired behaviour by allowing an Admin to specify CSS classes for the element in the form component edit interface.
It stores the CSS classes in the extra DB column, under the array key 'css_classes' (this could be changed I guess to perhaps it's own column, but it worked well for me).
Hope it proves useful!