I have been busy playing Braid, which is a great game that just got released on the Xbox Live Arcade. Therefore, I haven't really put much time into the procedural planet.
I did put in some code that would always display the calculated angle between the camera and the terrain mesh in order to see if that was causing the disappearing errors. I actually believe it is, although I'm not quite sure why.
First, I was surprised to see that probably a quarter of the time, the result was NaN. Even more surprising is that my terrain works perfectly even when the result is NaN.
The errors occur when the result suddenly comes back as pi randomly. I should never be getting an angle that big back. Heck I shouldn't ever be getting pi/2.
I will continue to investigate this problem (when I have time between Braid of course).