Some people have reported the “Choose files to upload” button being broken when trying to add an image to a WordPress post. The behavior I witnessed was that clicking the button simply did nothing. I have also heard that PC users see a blank white window. The quick fix is to use the Browser Uploader instead.
We’ve noticed the issue since upgrading to Flash 10. It seems the deadly combination to break the WordPress image uploader is WP 2.6 (and maybe lower) and Flash 10. Sites that have been upgraded to WordPress 2.7+ work just fine with Flash 10. See my previous post, What version of Flash Player do I have? to check yourself before you wreck yourself.