In ubbt.inc.php, find the line:
// --------------------
// set the style sheet
$stylesheet = "{$config['styledir']}/$fstyle.css";
Under this you can add:
if ($fstyle == "pdacss"){
$tempstyle = "pdatemplate";
}
Now if someone selects the style pdacss.css, UBB will pull all the templates from /templates/pdatemplate instead of /templates/default. With a little more code, it could auto-select the PDA style if it detects that the user is on a non-standard browser. However I'm not sure how reliable that would be on all devices.
In this manner it's possible to create a totally stripped down layout and still have the control which templates give you (unlike a solution involving the print thread mechanism).
As far as creating the actual stylesheet and templates... I have my hands a bit full with upgrades on my own site at the moment but I might take that on at some time in the future if someone doesn't get to it first

.
-Y