I've actually had a discussion about this starting here. Some say she could've read the note because JR laid out the note in the middle of the floor, but on the 9-1-1 call, it sounds to me as if PR had those four letters memorized. If she innocently read them for the first time, I would expect her to read it like, "S... I think that's a B... T...," and so on. She didn't.
Whether or not JR read the note behind the table near the spiral staircase in that diagram linked above or not is also important, IMO.
Ya, and who signs a ransom note anyway? And why would Patsy be the one to call police when she hasn't read the note? Their may be important information there. so why not take the 30 seconds to read the note? In my opinion it was the Ramseys way of ignoring the threats in the note without having to explain why. Patsy called 911 despite threats to behead her daughter if she did so, by claiming to have not read the note she does not have to explain her reasoning for ignoring the threat. Possibly she was planning on discarding the body later, so the unheeded threat would have been the justification for the actual killing.