I finally have a tutorial on MediaWiki Skinning for 1.18+ online on Redwerks' Blog. http://blog.redwerks.org/2012/02/08/mediawiki-skinning-tutorial/
The tutorial covers the general conceptual areas of a skin, laying out the files for a skin, all the small boilerplate pieces used to output types of content, an overview of things you should test for in your skin when implemented, a bit on i18n and related info on variants and rtl, and a touch on accessibility.