One of the nice features in FogBugz is the ability to type "case 17" in the body of a case, and have it automatically create a link to case #17.
If it's possible to extend MarkDown, it'd be great to be able to simply type "#17" in the body of a question or comment and have it render as a link to question #17 on the site.
It'd be even nicer if "#17" was automatically expanded to the full title of the question, linked to that question.
This would be especially handy when reporting duplicates - you could simply type "duplicate of #1234" and have SO build the link for you.
I'd like to be able to link to SO/Meta questions from SF and vice versa via a small link.