I'm having almost the exact same problem (hasn't affected my posts, only comments).
I'm using the latest version of WordPress and the latest versions of all my plugins (Akismet 1.15, Flickr Widget 0.1, Google Search widget 1.0, PHP Markdown Extra 1.0.1, SmartyPants Typographer 1.0, Google Sitemaps 2.7.1, Time Zone 2.1, WordPress Database Backup 1.7, and Sidebar Widgets 1.0.20060711).
This only happens to me on comments, and only when I edit them. The strange thing is that when I submit my edited comment, I get a black page with a confirmation ("Are you sure you want to do this?")--with no WordPress header/footer or styling at all--and a "No" link and a "Yes" button. I don't remember seeing this confirmation page before, and I'd bet that the form values are getting improperly escaped on this page.
I cannot see any way to get rid of the backslashes except to hack together my own form or edit the database values directly.