What do I wear to my friend's wedding

The question of what to wear to a wedding is often a vexed one, especially for women.  Often people have strong views about what is appropriate.  See what the media had to say when Kate Middleton recently wore black to a wedding.
http://www.catwalkqueen.tv/2011/01/kate_middleton_5.html

For what it is worth, one observation I would make is that often women wear clothing which though it might be "in fashion" is totally inappropriate for their age, their figure or for the weather.  A woman shows her class when she clearly appreciates what is flattering for her body type and her personality.