I have WordPress MU installed across multiple load balanced servers. The way they are set up is that file-based content must be uplaoded to the first 'primary' server and then is automatically mirrored to the other servers.
When the blog backend is accessed the request may be served by any of the servers. If an image is uplaoded when a user makes a post at the moment the image may be uplaoded to any of the servers. If the image is created on any server other than the primary server it will automatically be removed.
Any advice on how to ensure the images are uploaded to the primary server? In the past we have solved this type of problem by having a separate address for the backend which is always served by the primary server. Wouldn't be very scalable though.