I'm also waiting for some answer.
I tried every solution I found. I tried different browsers, clearing the cache, changing the press_this.php file... nothing works.
What is really annoying is that I had no reason at all to upgrade except that everywhere I looked there was a "Upgrade to 2.8.6" button that wouldn't disappear.
So wanting to get rid of the damn thing I upgraded, only to be greeted by a white text/white background problem.
So far whenever I had a problem with wordpress (and b2 a long long time ago) I found solutions rather easily. This seems to be an exception.
I cannot use wordpress like this and to be honest I'm quite disappointed. Did not expect this from wordpress.