Is it even allowed to charge for open source software in Ubuntu's Software Center? All the pay-for-it apps I found there were closed source.

Now that you mentioned it, it would be excellent if you could use Bitcoins to pay in Ubuntu's Software Center. I'm currently considering buying Braid, but I haven't found the time to figure out Ubuntu's payment methods. Also, I want to buy on the Android market using Bitcoins and I want a pony!