I really don't think that Petunia is a squib. Why? well because then she would know all bout he-who-must-not-be-named and would be afraid of him. She would be willing to protect harry, especially with Uncle Vernon in th ehouse. Maybe I'm wrong because she refused (in the fifth book) to kick harr out of the house.....who knows
or...remember in one of the books when neville was talking about how happy his grandma was when he fell out the window and caught his self before hitting the ground...using his magic for the 1st time...being that it was strong enough...i wonder if Petunia didn't have strong enough magic to get accepted into the school...like Lily was....b/c she did talk about "how happy our parents were the day she got her letter, but i was the one who saw her for what she really was...a FREAK!".....
so i think its all anger and resentment b/c she didn't get a letter.....