When I started using Sublime Text 2 as my primary text editor, last year sometime, I created a build system to more quickly process my MultiMarkdown files. Since I couldn’t find a preexisting MultiMarkdown build system in the Sublime Text forum, it’s probably a safe bet that others might find it useful for me to post mine.
Since all of my writing is based on MultiMarkdown and varying CSS files, I use the same build system for screenplays, blog posts, presentations, etc. When I created the initial build system I was doing the majority of my writing on a Windows 7 machine. Since that time, I have retired all of my Windows computers  and created a new build system for OS X (10.7.3).
Here are both build systems:
"cmd": ["mmd \"$file\""]
"cmd": "multimarkdown -b \"$file\"",
The last line in the Windows build system is a launch command that will open the processed document in your default HTML application. Since I have Marked on my Mac, I decided to omit the launch command from the OS X version and pick my viewer on a per-file basis.