C.C.
My 5 year old has become bratty lately too. I think she picks up a lot of it from school, actually. She was in Kindergarten this year and I noticed that a lot of her friends were allowed to watch Hannah Montana, Sponge Bob, and all kinds of things that I just feel are inappropriate for her. So even though my daughter doesn't get to watch that stuff, her friends do, and she picks up on their behavior. It seems like she wants everything to be a battle lately - from refusing to dress herself to complaining about which bathtub she gets to use at bath time, to whining about what's for dinner.
All I've really been able to do is send her to her room when she starts acting up. She cries very melodramatically, but after a few mintues, snaps out of it. I am really hoping that it's just a phase, because I'd thought that I had until she was at least 13 before she started acting this way! Ugh! So... you're not alone, although I'm afraid I don't have any words of wisdom. =)