I find it tacky - but then I find most wedding registries tacky. Unless you are right out of college/HS what could you possibly really need? I get that kids getting married might need stuff but older people/second weddings - ummmm, no.
If my SO and I ever get married, PLEASE don't give me anything! I have too much crap already. Ok, you can give me hay but I doubt I'd put that on a registry. And chances that I get married are small.
Back to the OP - I wouldn't go and I wouldn't donate money. I might send a card with regrets saying you sent $$$ to the red cross to help with hurricane Irene relief.
People gossip about how tacky all KINDS of things are. It's just that people make such a BIG DEAL over weddings that it hypes up the tendancy of invitees to gossip about it. Trust me, there's plenty of things I find tacky that I'd happily discuss on a thread if someone starts it.
I'm pretty sure I will not attend the wedding - it's on a Thursday. Sautrday or Sunday possibly I'd go but Thursday is a working day and I'm not taking a vacation day for the festivities.
Not sure what I'll do but will probably send some money and a card.
Or do what I did for my niece's 11th birthday this year. Since she can't be bothered to acknowledge a gift, I made a donation to a no-kill animal shelter by her and had them send her a notice of the donation and what it was for. Still didn't get any acknowledgment from said niece I felt good about it and now I can deduct it on my taxes