Assumuption of numeric key, limits flexibility

A row key doesn't necessarily need to be numeric; for example the invite module uses a varchar(8) as a key.
However in the validation function it uses array_sum to see if there is any selected rows, which returns false in the above case.

     if (!array_sum($form_state['values']['objects'])) { // If all 0, no row selected

Attached is my solution and seems to be working, at least for the above issue. It's against v1.7 done in the vbo folder.