Last Friday I met Aaron and drewish at the Ace hotel for a day of SimpleTest writing for the long-awaited hook_file() patch. This was part of the Portland Media/Files Code Sprint, organized by drewish to solidify the patch during the week of OSCON.
The day was very productive, as file.inc went from having virtually no tests, to near complete test coverage. As is often the case when writing comprehensive unit tests, several inconsistencies with expected behavior/documentation were found and fixed.
Aaron has provided an overview of the goals of the code sprint, and the remaining work to be done (both on the immediate patch, and beyond). But to summarize, the goal is better file (thus, media) handling in core.
If you maintain or develop modules (or themes) in Drupal that need to deal with any sort of files, go give this patch a try (it's currently the spotlight patch).
I've had a tremendous amount of fun writing tests for file.inc (hopefully I'll get to do a few more before the patch goes in), so I may very soon jump on some of the other needed tests for Drupal core.