If it's a big charity I believe in then I'll do it (Tour de Cure). I'm not going to hit up all my friends for donations to something I don't feel particularly strongly about though... I'm sure you've noticed as soon as you hit people up, you're fair game for their charities - it's like selling stuff at work (cookies etc) I don't do it.
There's a ride near me I'd really like to do but it's got a $3k fund raising requirement. Oy!