The current mobile site (e.g., http://en.m.wikipedia.org/) is written in Ruby. Having the site in Ruby causes a number of headaches (for code review, code deployment, and server maintenance) given Wikimedia's mostly PHP-based infrastructure.
I've created a page at http://www.mediawiki.org/wiki/Mobile_site_rewrite for notes about this.
Version: .5
Severity: normal
URL: http://www.mediawiki.org/wiki/Mobile_site_rewrite