Both Squid and Varnish support edge side includes, but support needs to be added to MediaWiki in order for us to take advantage of this. If implemented, MediaWiki could send out sections of a page like so:
<esi:include src="http://en.wikipedia.org/wiki/Special:CentralNotice?action=gimmestuff" onerror="continue"/>
...and they would be cached separately from the rest of the page.
This would open up a lot of new feature and optimization possibilities for us.
Request for Comment page can be found at http://www.mediawiki.org/wiki/Requests_for_comment/Partial_page_caching