I'd like to third this... but to not even call it AME, but simply a content regex find and replace for URLs... yes it could do media but I want it to also do affiliate schemes and skimlinks style things.
What I want:
1) A database table of rules defining what to find and how to rewrite/replace it (find URL, replace with embedded media).
2) A hook that reads post content and if a URL exists will apply the rules.
3) For the above to be open source hosted on github allowing others to work on it.
4) For the above to be done so with a high-volume site in mind. I.e. to memcache the rules and to take advantage of any Xenforo post-parsed cache that appears in the future.
5) For a cron job to load Skimlinks rules from an XML file.
Actually... I may create this if no-one else does.