I think the real definition of a killer application is when you'll purchase the platform just for the app.
I think the new version of Iotum's Talk Now might qualify here, as I do not own a Blackberry, but am getting to the point where I would purchase one just to use Alec's service. Iotum's talk now takes presence from a number of connected users, and allows them to express the desire to speak with each other, and the presence to tell them when it's possible. Essentially, when you use this service, telephone tag goes way down, along with interruptions from those people you do most of your communicating with. Get the details here while I think of some way to convince my wife not to kill me for buying three phones in four months.