I would have a simple method for getting the code in plain text, but I wouldn't bother including dependency handling natively. I feel like all dependency checking could be done with extra code in the snippet and handled on the client side. That would be simpler and more open to changes in the future. Plus we don't have to spontaneously create a system that is both simple and comprehensive enough to satisfy everyone simultaneously ;)
Basically I would extend the news.arc code to include support for another item type - code - that can be submitted separately, named, searched for, and retrieved as plain text. Then all of the comments/discussion is included for free, and if the double indent was made slightly more intelligent it could extract code snippets from normal comments as well.
It doesn't seem like it would be too hard to implement this way (most of the code is already there) and if we can get pg to like it he might update the arc forum to support it ;)