I got invited to go with a friend to a wedding this saturday - it is a big deal - it's pretty much the who's who of the dallas music scene - it will be major networking for me. the link for the wedding page is www.chanceandkristi.com they say "informal dress" on there.
Informal is a tough description. Do you know what the bride and groom or bridesmaids are wearing? It would be telling if the groom plans to wear khakis or the bride is wearing a casual dress.
I just went looking all over the DC metro area for a dress this past weekend, and I have to say, BCBG had the best selection of any B&M store.
Another option is going simple, like a black dress, and get a funky necklace or pair of shoes.
Here's another dress I love, but didn't have time to order it: