Hodgeman - I'm not surprised your game already felt smooth because using a fixed time step that is equal to the vsync is the ideal case to not have jitters. Its interesting that tried that method to force it to produce duplicate frames and still couldn't really detect it visually. It does depend on the type of game, for some games it is less obvious. The most obvious case is a 2d scrolling game where pops like this are very apparent. In 3d fps type games it seems less obvious. I also think that many of even the very best games have this jittery issue so we are trained to not notice it.
I think the whole point of triple buffering is to allow the CPU to fall behind a bit to smooth out pops by sacrificing latency. So even if you have just 1 new backbuffer to display and the vsync happens then it will flip it, why wouldn't it? The next vsync you will just need to do 2 frames to catch up. This is how pops are smoothed out even if you have a very long frame. Triple buffering + vsync is a way of actually decoupling the vsync from the game update. Even though you get 1 update per vsync on average, it's really more about keeping the back buffers updated then waiting for the vsync.
Icebone - Wow, that's a really cool idea! Keep in mind though that due to rounding that gaps in the pattern may actually be correct. So for example if it's moving at 1.1 pixels per vsync, you would expect it to jump an extra pixel every 10 vsyncs. The math to make things move an integer number of pixels when there's a fixed time step with interpolations is a bit complicated. What I do to visualize jitter is have a wrapping image scroll continuously across the screen. When running at 60 fps it should move extremely smooth. I will need to think more about your idea though, I like how it is visualized over time.