I have an idea about an app and one of the many things it would do is allow the user to view twitter and facebook updates in the same stream. This really isn't key to the app, but I'm wondering if this is something I should include or not? Maybe you could choose whose updates you see on facebook or twitter.
Any thoughts on if people would want this or not?
That's already there in TweetDeck
It's difficult to answer the question on the value of adding that to your app, without knowing more about the app you are creating. If you were creating a IM client, than I think you have to provide Twitter/Facebook integration to stay competitive. On the other hand if you are creating a Notepad replacement, it's not as relevant.