The twitter app I built http://tweetarium.com would benefit from something like this. With regards to twitter pulling the plug on third parities, while it is a worry this could happen, it's risk you factor when building on any platform.
If at all possible try to differentiate and make your application work with other ways of exchanging messages (such as IM).
That way you're not completely dependent on just the one platform. Ebuddy is a nice sample of a company that did that right, they started out as single platform and now support a whole bunch of them.
