Here's a PageDown plugin I put together based on the WMD plugin source. In editpage.tmpl I moved <TMPL_VAR WMD_PREVIEW> to the top of the template. Makes it look like the edit textarea pops up below the content when hitting the edit link. Should work the same with the WMD plugin as well.

I also wrote a couple of makefiles to make fetching and installing the sources for the pagedown and wmd underlaydirs simpler. And updated doc/plugins/wmd.mdwn to reflect these changes.