Nick, the last statement,
"While this is correct for most purposes, it does mean that..."
can be simplified to, 
"It means...". 
I had to read it several times before I realized, there is no "not" after "does" :) 
BTW, since this particular arrangement of having a temporary thread state during the callback is particularly useful for ctypes (I cannot imagine any other usecase) and also it sort-of breaks things, a potential feature request could be to have consistent thread state during the lifetime of a C thread. I don't have much idea how to do that or whether it is even possible? Would anyone like to give a thought?