BTW the wiki can be searched and read by adding your search terms
here and clicking on the "cached" links instead of the main links in the search results.
I guess it's a bit late if you've already rewritten MediaWiki in Ruby but check out
JAMWiki, a Java-based reimplementation of MW. The dude writing it is pretty hardcore and is following very good development practices; his intention is to create the premier wiki software that would be used by corporate audiences. He's got login working through Spring Security so you can already do crazy things like have your wiki users authenticated through an LDAP directory. Lucene for search... he's aiming for maximum MW compatibility and he's already got basic templates functioning very well.
The default is for it to run with a SQLite database or something file-based like that so you don't even have to set up MySQL if you don't want too (and then you don't have to set up database backups, a file backup gets everything, which suits me perfectly for my small wiki site.)
I run JAMWiki on Resin Server, which includes their Quercus Java implementation of PHP 5.2, so it's pretty cool - I can mix JSP pages and Java apps with PHP pages, many major OSS PHP applications run, but I can call Java classes from PHP if I need to.