I've thought of this before.  Another way it could be done is Bitcoins could be donated to the charity (or a dedicated receiving address for the charity run by a trusted party).  Then someone who is feeling charitable could donate USD/etc to the charity in an amount to cover the coins at market value.  When the charity confirms receipt of the donations, the coins could be handed over to person who donated USD/etc to the charity.

A good way to do this would be to find a worthwhile charity that Bitcoin users would be likely to support.  Get them to agree to this fundraiser, and then we can all chip in a little something.  It would be great for the charity, and good for Bitcoin as well.  A way for Bitcoin users to "give back."