Sencha documented their effort in making the Facebook app and were very clear on how they managed to get smooth scrolling with their infinite implementation.
It started with the implementation of an Infinite List Component that handles items with unknown sizes. Only a very small set of DOM nodes is actually created to fill in the actual visible screen area. They will then be constantly recycled to render next / previous data on-demand. As a result, memory footprint is kept minimal, regardless of the amount of data in the Store. Making this work is the easy part. Making it fast with the complexity and variety of items such as News Feed stories is the real challenge. The bottleneck lies within the core processes that a browser has to perform: layout and compositing.
Since we are aiming for the Discourse app to work across a multitude of devices, would it be worthwhile putting some effort into this? This might help with the scrolling performance on large pages too… (it shudders at times on my MBP when on HD3000 graphics.)