I just noticed this a few days ago, but I suspect it happened with the last Jetpack update and I have simply missed it. When Jetpack is activated, all featured images are broken. It looks correctly in the Edit Post page, but the images do not show on the Blog page or the actual published/previewed post page. I haven't had the chance to test if it's just a certain feature in Jetpack, or if it's the full plugin itself.
The error started on 3.5 - I updated to 3.5.1, as well as updating my theme hoping that would fix it, but it does not. I've turned off plugins, but this seems to be o
I currently have Jetpack turned off and the featured images seem to show everywhere. (As you can see - http://www.PenandMuse.com/blog )