S3 is seemingly costing us a lot. While it has its benefits, I think we could do with moving images from s3 to the server we are running the site from. How difficult/impossible of a task would that be?
The easiest way I imagine would be to
And you're done. Whoa, that was easy :)
Ignore my answer. Andris' is better.
There is no functionality in Craft that I'm aware of, but you should be able to handle it similar to moving a website. The hardest part I think would be updating fields and references within entries.
There's really no automated way to update fields and entries, but it might be possible with a really clever plugin.