Just got a solution for select fields that can also be applied to checkboxes.
The idea is to add a "|<value>" (vertical separator + value to insert in database) inside the label text. So, if you just want to keep a "1" if checkbox is selected in case someone wants to subscribe to your newsletter, simply write the following stuff:
[checkbox checkbox322 "mark me if you want to get newsletter|1"]
The original info is available at http://contactform7.com/selectable-recipient-with-pipes/