I've been working on this on and off (mostly off), for a few months now. Finally I got round to finishing it to a reasonable state! It's a Markdown editor plugin, which has several advantages over CKeditor, the default in Q2A.
More details on the plugin, instructions and files to download are available here: http://codelair.co.uk/2011/markdown-editor-plugin-q2a/
It's licensed under GPL so it's free and open source. There are some minor bugs that I haven't managed to fix just yet, so if anyone manages to fix them please let me know and I will update it. Also, I have copied the files direct from my Q2A site where it's been in use for the last little while, so if there turns out to be any site-specific code that I left in there let me know ;)
I'd love to know what you think, and any suggestions for improvements! Just post an answer here.