<p>See title. By lightweight markup I mean something like markdown or wikitext.</p>

Well, silverstripe
http://www.silverstripe.com/
is thought to be a good option for people who care about web standards , as is modx
http://modxcms.com/
For a lighter editor, check out textpattern.com which uses textile (like markdown). If you're on rails,
http://webby.rubyforge.org/
might do the trick.
http://www.madebyfrog.com
is a port of the ruby CMS radiant that aims to be fast, light, and minimal.