I'm really happy to announce that full screen search is out in out beta: http://blog.wikimedia.org/2011/12/02/mobile-full-screen-search/
This is a really exciting change for our community as its the first significant design change that we've made to the mobile site.
Many thanks go to Patrick and the rest of the tech team for getting this out.
If your on this list and not in the beta then head over to http://bit.ly/wmoptin and then tap on search to see our changes.
--tomasz