Hey Chad, I wonder if Apple would allow a Gittip app using in-app purchases? They just released subscriptions too. That way every downloading user would already have his CC on file, and be a click away to donate.
The idea is that the volume makes up for the tax - 100% CCs on file vs 4% is huge. Not the first thing you'd think of for supporting open-source, but something to consider.
Consider using Dwolla.com instead of credit cards. No transaction fees at all for < $10. And it's run by a brilliant entrepreneur determined to disrupt the credit card industry. Cutting edge stuff.
