Forum OpenACS Development: Anonymous Donations
That's an interesting idea, though I'm not sure if it's possible using a credit card. If you wanted to remain anonymous, you may have to send a money order by mail - old skool but might be less traceable. ;)
As far as legislative reasons, I don't know.
Also, if they register in order to donate and then forget their password, if they want to come back and donate again they'll need to make up a new bogus email address or go through a password retrieval process in order to be able to give you more money (again, a point where you're potentially going to lose them and their money).
I'd suggest that you record the donators info if they're a logged-in user, but also allow users who are not currently registered or logged in to donate...
Of course there's emerging technologies that make real anonymous payment methods in internet possible and I think we should even follow how these things evolve.