I'd be interested to see that too. I can't seem to do it with my version of FB (which I believe is the latest). I can choose to (universally) change a field 'type' but not the individual fields, as Ray suggests.
While I'm here: Has anyone had any success in significantly reducing the width of form or a text area? Setting the width in the UI seems to make little difference (either selecting 'small' or setting a width/px). The mechanics of the UI and its effectiveness is a bit of a nightmare at the moment.
Supposing I want a form that will fit in a 250px wide column - as far as I can see, this can't easily be done with form-builder. All the FB forms seem to require a near full page to display, even after much fiddly editing.
I find that trying to edit a FB form to present as I would like it (have yet to manage it) is far more time-consuming than creating the form with the standard Dreamweaver insert options, and custom stylesheet which you can then save to your own 'library'. You get more attractive forms, can more easily control individual elements and stay true to the site style.
The validation facility is really useful though, and I use it for any forms I make.