Confirmation and update:
At the 2012 district convention just concluded in SW Ohio, USA, there were 6 credit/debit card machines in use at different places around the site. They were wireless (I presume there was a wireless router somewhere on site) so all the volunteer 'watcher' had to do was find an empty table and set up their sign and printed directions for use and put the device out there. I observed a steady stream of people donating in this manner. There was no minimum donation (I asked).