I also suffer from this problem many people have these days. Every day my recently opened second blog with Ocean Mist theme (http://cliffmodes.wordpress.com) resets my custom header and tells me that my created pages were not available (error 404 - file not found). Then again, it may change to full working mode again in the evening before being resetted the next afternoon or so.
My main blog (http://caveras.wordpress.com) is not affected by any of this. What's wrong there? I'd really appreciate your help! =)