Translation of a post’s text into leet speak can be accomplished by using one of the following methods.
With the first approach one can target specific parts of their post and translate them using the [leet] and [/leet] tags.
The second method allows for the translation of an entire post into leet speak by setting a custom field of leet = 1.
http://dan.doezema.com/2010/04/leet-speak-plugin/