I'm having a similar problem. I can edit pictures that are already on my site but I can't add any new ones. After I upgraded to 3.0.1 I am no longer able to insert images into posts or pages from my library. I *can* insert images "From URL" but not "Media Library". Everything worked fine prior to upgrading.
I checked the uploads folder (images are where they need to be), I set my theme to the default and I disabled all of my plugins and it still will not insert images from my gallery. I even tried using various browsers such as FireFox, IE, and Chrome. Please help! Also, I have never edited any code or tweaked anything..I'm just an end user of WP.
I get this error:
Error 404: File Not Found
The requested page is not found. This may happen due to the following reasons:
Page or file is outdated, renamed, moved, or does not exist.
You typed the address incorrectly, like http://www.example.com/pgae.html instead of http://www.example.com/page.html
Please contact your webmaster if you are not sure what goes wrong.