I do about 95% or more of my NB time on my iOS devices, primarily an iPad without any problem. Those that know me, know that I can write long structured posts. For these, I almost never need to go to my computer for. I'm simply not sure what specifically an app would add over a web browser interface unless it was refined for this site type, which would take time and effort that I'm not sure people have. What's the cost/benefit to the actual programmers here, not the end users? I think that's an important question if not the critical one.
Another unmentioned annoyance is that an app would have to go through approval of the app stores' approvals, which likely vary to an extent per platform. This would have to be successful before dissemination of an iOS app for certain unless the device was jail broken.
Further, I would think a web app might be the best language so that a separate android and iOS version might not have to be developed, especially considering that the Windows 8 platform is being introduced this year potentially further fragmenting the tablet/smartphone OS programming language.
I'm sure an app with message and thread notifications API, amongst other features would be great, but who's going to develop it, and how much better will the experience be compared to the cost of developing and maintaining it?
Peace