I haven't seen anyone on this thread mention it, but this book doesn't seem to shed much light on the significance of the 'Remember my last...' message from Dumbledore to Petunia in OOTP... there was mention of her writing to Dumbledore as a child, and him sending a polite reply, but 1.) that wasn't really his "last" correspondence, because he did leave a letter with infant Harry, IIRC, and 2.) even if it was, what would that have to do with the context of throwing Harry out of the house in OOTP?
I guess it must have been a bitter pill to swallow that Dumbledore wouldn’t let her go to Hogwarts but then wanted her to keep Harry.