bonch writes "In an attempt to tackle the inefficient complexity of its current template system, Wikipedia will be adopting the Lua scripting language. Known most for its use in videogame scripting, particularly World of Warcraft, Lua is lightweight and designed for easy integration into existing applications. The transition is expected to begin after the release of MediaWiki 1.19, possibly in May."
Basically, the template system started turning into an ugly programming language
; Lua ultimately won due to implementation concerns. The mailing list threads announcing the decision
and discussing the change
have further details.