I had the same problem today – I upgraded my site to WP 3.0.1 last week, and posts that I wrote a few days ago uploaded fine. Today however, no luck. Found out – thanks to the great folks at Dreamhost – that it is indeed related to the .htaccess file issue
There seems to be an issue where WordPress reverts back to PHP4, which is
causing the errors. We have posted a status entry on our blog:
http://www.dreamhoststatus.com/2010/11/18/php-errors-related-to-uploading-files/
I made a slight change to your .htaccess to force PHP5 to be called so
hopefully your WordPress won’t be acting up when you upload a file!
So update that file if you can or contact your hosting provider.
