IMPORTANT: The accepted answer was accepted post-bounty, not necessarily because I felt it was the best answer.
I find myself doing things over an
I use a similar layout, but with one major exception: all of these directories live under a top-level media/ directory. This is for a few reasons:
A custom template tag (I have used this with 2 Django projects, but you could do it in PHP, etc.) generates urls which a) semi-randomly choose one of the media servers, b) add the time-based pseudo directory to the path, and c) give the object an Expires time of now + 10 years.