Generalize for any select form element

I think that this would be great as a general form element like #select itself. One could themn use hook_form_alter and change #select t #multiselect when you wanted that UI. A #process on the field element would transform the input data so that any validate and submit handlers would see the data just as if a regular #select had been used.

I would use such a thing in organic groups module for selecting audience.

Basically, this sor tof generalization is going to be needed anyway to port this module to D6 becuase cck widgets are now just regular fapi form elements.