Movisol is offering Your Ideal Weight over at the Windows Phone 7 Marketplace. The Windows Phone app estimates what you should ideally weigh, based on your Body Mass Index and the preferred weight, based on height and age. According to the Marketplace description, the app then uses both to calculate your ideal weight.
The developer cautions that if you are a child, a bodybuilder or pregnant, this app will not give you accurate advice.
Your Ideal Weight is a free application and you can download it here (opens Zune) at the Marketplace.