In two words, very rude. Emergencies such as a violent illness or a death in the family are pretty much the only excusable exceptions. Failing to show up at a wedding you said you would go to is so rude you can pretty much expect an angry phone call from the happy couple. Besides, would you want someone doing that to you at your wedding? Catered dinners at the reception don’t pay for themselves, you know! Figuring out who is attending, what they want to eat, and how much alcohol is needed are some of the most stressful parts of wedding planning. Dealing with a no-show will subsequently cause turmoil and anger on what is supposed to be a very happy day.
If you know ahead of time that you cannot attend the wedding for whatever reason, let the right people know. They will probably be miffed, but at least you took the time to explain your reason. It also helps to send a gift from the registry–this will lessen the blow somewhat so you can keep your friends.
Skipping out on an event you said you would attend–no matter what the event might be–is considered extremely rude. Period. Don’t be one of “those people”!