I just ran across this which looks like I can potentially cleanup one of the annoying issues that the S3 implementation that Media Mover uses.
Currently, when Media Mover goes to push a file to S3, it has to run the full file through memory, demanding a much higher memory overhead each time you want to move a file. This can mean going above 100mbs for PHP which is not exactly possible on shared hosts.
I'm going to try some implementation with it and see how things go. Hopefully this will significantly improve S3 performance.
Update: I've started the process- http://drupal.org/cvs?commit=156228">CVS commit
Update 2: This is now integrated on D5 and D6 branches