We did get one christmas card last year that said that they would not be sending cards this year, but would be making a donation to a charity (I think it was for MS) instead. Sounds good to me! We did that in work too - everyone gave a couple of pounds and we gave it all to a charity instead of giving out cards.
Why do people have to know one is donating to charity except to inform them what a Wonderful Person one is? Why not send cards as usual and donate to charity? Or send "charity" cards? I normally send cards to people I don't see very often; it just means you've remembered them and value them. Some, but not all, of a group of pub mates have taken to sending (i.e. dishing out in the pub) Christmas cards to each other, which I think is barmy. I don't do it and it's done me no harm whatever.