We chose Pottery Barn's Organic Tile Jacquard Shower Curtain (no longer listed online, but available on clearance in some stores, marked down from $60 to $40).
We hung the curtain on a stainless steel rod with clip rings. Unfortunately, we forgot that shower curtains are a lot wider than standard panel curtains, so we'll have to go back to the hardware store for more clip rings.
After we hung the curtains, we decided to do another sheer white layer underneath, so that will be our next window project. We like the way it looks so far, and since the curtains are made from organic cotton you can't really tell that they're meant for the shower.