After we upgraded to Wordpress 3.0.1 (from Wordpress MU 2.9.2) we began getting an error message whenever we tried to upload media.
"Missing a temporary folder"
Various articles around the web mention the php setting
upload_tmp_dir needing to be set in
php.ini. This is not actually a requirement. I know because it is not set in our test environment, and it was not set in our production environment before the deploy. In both cases uploads worked.
So what changed?
I have an answer, but I'm not sure it's the definitive answer. I'll post mine.