A very important concept in the evolution of languages, metathesis allows us to understand why we say "Wensday" for Wednesday and ax instead of ask. Also, the Anglo-Saxon originators of the word bird started with brid. One used to pronounce clasp, clapse, etc.
Just a good term to keep in mind when certain people start messing with your pronounciation. I look at the concept as a sort of path of least resistance while pronouncing a given word. I just feel that these days standardized spelling constipates the english language.
See also: busker (what got me started on noding this already Webstered idea)