This plugin by ereckers sets uploaded media filename to lowercase as filter on sanitize_file_name. Based on post at Stack Overflow asking if there is a way to “Rename files during upload within WordPress 3.0 backend”.
Created for WordPress 3.0.1+ which saves any uploaded media filename (Image, Video, Audio) as lowercase. This effectively changes the name of the file at upload if a user were to upload a file with any uppercase characters.
Download and info: http://wordpress.org/extend/plugins/make-filename-lowercase/