Since learning about how Radiohead did this, I have been thinking about this alot. Would this model work for software? If you had to pay _something_ would you just pay $20? $40? $1? There are a lot of little utilities I use that I can see paying $20 for.

$20 is the magic number. If it's just a donate button, I'll probably pass. Honestly. Who wants the extra work of pulling out the card and filling out a form. Not me. Better things to do. If it was just part of the process though, and especially if it used something like amazon.com, where I don't have to fill out any forms and they already know me, then that's just part of the price and I'll gladly pay $20 if it's something I want.