15 Nov Proposed change for ms-files.php & image handling
This trac ticket has been tagged for 3.4 which is quite a while away from happening. Basically, the current way multisite handles images on the sub sites, is by running every request through a file called ms-files.php. This can actually be overridden via the sunrise.php file.
The proposed change would involve moving new sub site images into wp-content/uploads/sites/ where it makes much more sense. I've seen a lot of confusion over the blogs.dir folder and image rewrite rules.
This would also be handled in such a way that your current image uploads would not suddenly break. That would be a nightmare.
If you have worked extensively in this are of multisite, please chime in on the trac ticket.