Solution: create an account at GitHub, it would have taken less time than writing this comment ;)
You probably won't log out, and chances are you'll end up on GitHub again sometime soon. You'll discover the wonders of social coding now that you have an account, and soon enough, you're a paying customer.
But the point is not how easy is it to sign up for a free GitHub account. The point is that it seems like an unnecessary requirement to tip someone on Gittip.
https://github.com/whit537/www.gittip.com/issues/80
Trying to get the payment flow nailed first.