Yesterday, I attempted to change over this installation of Symphony from using the Simple HTML formatting, to the Markdown formatting idiom. I did this mainly because I needed to be able to tweak entry HTML as much as I needed, and the Simple HTML was 1) not formatting what I wanted it to format, and 2) constantly creating XML validation issues. Despite the lack of much information about installing Campfire Services (who reads the Read Me file, anyways?), all you really have to do is download the package, upload it to the campfire directory, and to Campfire > Your Services in the Symphony menu. Then you just need to follow the instructions to install the service.
Once it was running, I had to figure out how to get it to apply to my entries. Formatting preferences are saved per author, so to change it from Simple HTML to Markdown, click on the Formatting Preferences select box, and pick Markdown Text Formatter. You can also use the Extra or the Extra with SmartyPants, but I haven't tried those out yet.
After my preferences were saved, I still had to figure out how to use Markdown. There's a great syntax guide here. I was initially confused when the updated preferences didn't apply to my past entries, but apparently Symphony saves formatting preferences per entry, on creation. It makes sense—that keeps stuff from breaking if you change preferences, but it would really be nice to be able to change formatting on entries once they're created.