In fact, I just realised that I really like Ubuntu and would happily make a monthly donation to support it (and can afford to) but I've never seen anything that suggested I do so.
I can't find any hints on ubuntu.com on how to donate (though they have a shop, I don't want any "swag").
Shuttleworth has always said he wants to make it a viable business; perhaps he's worried people would donate instead of having their business buy support (or whatever). Hrm.
EDIT: I searched their site and found the donate page. It doesn't seem to be linked from anywhere; only discoverable by using search.