I am reading the August 2009 edition of Yoga Journal and there's an interesting article about practicing yoga as a couple entitled, "Grow Your Love". I'm a firm believer in the healing and restorative powers of yoga for mental health, but I'd never thought of using to strengthen a relationship.
This really is a beautiful article, but I only have the print version. But with a little Googling I found an order page if you want your own copy.