Interesting Idea, but what about the tax implications?
I donate to charities annually; I like the tax credit.
I dont see anyway of doing this while donating with bitcoins...just a miner (pun) issue =)
Perhaps a good way for the bitcoin community to 'donate' to these charities - is to donate the time and tech skills to help set them up with a solution which allows them to give a receipt in local currency equivalent ( or optionally just donate anonymously).