The idea is to create a simple bitcoin mining client (and maybe a pool) that runs in the background and raises money for charity. Anyone could use this even if they had no interest in bitcoin.

The money would be converted by a third party and sent to the charities bank account (bit-pay seem the obvious choice).

I think it would create new interest in bitcoin, and give bitcoin some positive press, and we would have thousands of extra people raising money for charity and protecting the network at the same time.
Even my mother could take part in this as all she'd need to do is download the charity miner and pick her charity.

Any registered charity could sign up to the scheme and perhaps each user could pick the charity they wish to contribute to.