I dig the shirts. Glad Ubercart is working out for you, and I'm very excited about the possibilities w/ Workflow-ng, too. As for making attributes a checkbox instead of select... there is no simple way. I've seen some snippets floating around made by others that let you change attribute boxes. The code needed is a module that implements hook_form_alter() to change the add to cart form so the select elements you want to be checkboxes have their #type changed. For a checkbox, you really might have to write a bit more to make sure it saves right. If this is unfamiliar to you, perhaps someone else who's done something like this can post their code.