this is actually on my to do list as well.
i have chosen a sweater that i am simply going to knit longer and add a belt, but i like that kinda of sweater dress. you could look http://www.berroco.com/item_fr/fr_dress.html
or on ravelry to find something you may like. i have ten tons of vintage patterns that i tend to go through. i will have a look through and see if there is anything there that may interest you. i inherited hundreds of patterns from my gran and great grandmother.
There are masses of free patterns out there, you could google for free (if you dont want to pay for a pattern) dress pattern, knitted and see what you come up with.