My point is this: I very much don't believe in the cry it out (CIO) method of sleep training, but I do believe that crying can be a form of self soothing and only mama knows when the crying has crossed the line from one to the other.
When I think CIO as a form of sleep training I think Ferber, I think abondonment, I think a baby crying until he throws up and finally falling asleep because no matter the amount of crying - mom won't comfort him. When I think crying as self soothing I think of my tired baby fussing and whining in bed/crib (the same thing he does in my arms/baby carrier when he's tired) while he winds himself down and out.
I think moms know their babies cries well enough to know the difference between a fussy baby and a cry of fear and abandonment. In my house we call it the "momma cry" and I hear it starting right now.... gotta run and stick a boob in his mouth :)