Tracking this and the communion thread I think that both demonstrate something amiss in how we respond to big(gish) milestones in life. I'm as fond of the filthy lucre as the next person but there is more to life than money (no, really).
CD- how about writing the couple a nice letter saying that you are devasted you can't come, delighted that they have eschewed additional possessions and that, in the same spirit, you are making a donation on their behalf to a charity to spread the good karma for their happy day. They might get the message!
Ragazza- I've been in a similar situation and I really think a gift in that situation should be token only. Something that means something to them, has an association with a special place etc? (Of course if you are time poor, these gifts can be more troublesome than the big money variety, but it sounds like you are willing to go the extra mile for this particular wedding)