Caz, I also think it was a little harsh to describe our concern (which is all it is, after all) as 'arrogant'. Although I do understand the points made by Yopp and others.
Of course we can't change Mika's schedule! But we can show our concern and that we are thinking of him, just as fans, not because we think we are his friends I see nothing wrong in sending a message to him to this effect - even if he probably never gets it. In the same way one would express concern to a friend or work colleague if we thought they were overdoing things. It doesn't mean we expect them to take any notice..it's just an act of humanity!
I'm happy enough to just leave a message on his myspace, for what that is worth. There, I have done my bit. But Caz I don't think the card is a bad idea either if you are really into it.