A lot of people are having trouble logging into Pixar Planet Forums lately, and it appears as if the problem is caused by a bad cookies. All you have to do is delete your Pixar Planet cookies and voila! it should work again. I had an issue logging in through Safari 3.0.4 on my Mac, and others had trouble with Internet Explorer. It seems Firefox was resistant until now.
To find out about how to delete cookies in your browser, click here.