I have got a strange problem with the styling of my pivot control. I edited a copy of the default template in Expression Blend because I want to remove the entire header.
The template of the Pivot control was changed in WP8 and it now requires that the PivotHeadersControl be present in the template. (You could remove it in WP7.x)
Just have a zero height or other "empty" content in your header instead.
I'm not aware of this having been publically documented as most people who've upgraded to WP8 are using the shim to the old version of the control. However, I Noted this at the end of a blog article at http://blog.mrlacey.co.uk/2013/01/pivot-and-panorama-have-moved-and.html