I was surprised by both the location and the severity of this crack. I didn't expect this location to be such a significant stress point. You can see some scuffing on the edges where the chain slaps against the frame on bumpy trails. Here's a modified shot from Niner's website showing where the crack is located on the frame.

Aside from the headaches dealing with the frame failures, I have to say I love this bike. I ride it about 3 days a week year round, so it gets lots of use. I still wish it had another 1/4" - 1/2" of BB clearance, but I have no further complaints after two years on this bike.
I'll contact Niner on Monday and see what they say about the replacing the rear triangle (again). I'll post when I hear back from them.