To add to what Jim said, any extra data points from other machines would be appreciated.
On my home system, which has a good video card, the frame rate stays near 60fps but the scrolling sometimes _feels_ just a little strange since this change. It's like it jumps a little more than I'd expect (like maybe a frame dropped). However, I can't reproduce it reliably.
I also tested on a 2500k with no video card (HD3000), and it seems to be working great.
Thinking about it more, I wonder if flip presentation changes the number of queued frames? When we build a frame, we use the current time (build time) instead of the time when the frame will be shown (present time) since we don't really know the present time. If frames are being accepted / presented in a lumpy fashion, we might not be moving the frame time smoothly.
We'll keep digging. Like I said, any extra testing or data from anyone would be appreciated.