This post of Karl Shifflett might be of help to those of you who are writing their own desktop, phone and web applications that use forms authentication.
Source: Karl's Blog
I’ve been working on Stuff v2; a movie, game, and books application. Its primary use case is, "I’m at the store and don’t remember if I have a particular movie, game, or book. I need to determine if I have it; if not, then check the online price and ratings before making the purchase."
Given the varied application clients and devices, ASP.NET forms authentication seemed like the natural choice for authentication for the website, MVC3 JSON endpoints, and WCF services.