smiles, if they really have everything then they don't need money either so I think that a really appropraite gift would be one from the oxfam unwrapped catalogue.
You can order a wedding card with it

tehehehehehe


ETA -as for whether it's rude or not I guess that different cultures have different traditions so what is normal in one might seem a bit rude in another. In DH's culture everyone gives the bride gold jewelry. It seems a bit odd but it means that if something goes wrong and the woman wants out of her marriage down the track she has something of value that she can use to support herself while she finds her feet.
My Mum thought it wsn't fair in DH so she gave him some gold too lol.