Spoonerism
Spoonerisms are a kind of word or phrase pun that have humorously switched or transplanted sounds (see metathesis), named after the Reverend William Archibald Spooner (1844-1930), who often spoke these aloud. Some of his famous (and possibly apocryphal) quotes from the chapel include "The lord is a shoving leopard," "It is kisstomary to cuss the bride," and "Mardon me padam, this pie is occupewed. Can I sew you to another sheet?." The spoonerism is a now legendary 'slip of the tongue.'Other gaffes worth mentioning are his angry speech to a student, "You have hissed all my mystery lectures, and were caught fighting a liar in the quad. Having tasted two worms, you will leave by the next town drain," actually intended to say missed history, lighting fire, wasted terms, and down train. A few more which you can probably work out for yourself include "We must drink a toast to our queer old Dean", "We'll have the hags flung out", "a half-warmed fish" and "Is the bean dizzy?"
Proper Spoonerisms
In modern terms, spoonerism is any swapping of letters in this manner. While simple enough to do, a clever spoonerism is one that results in a funny sentence. The Capitol Steps have successfully done a few political comedy routines based on this premise. The comedian Ronnie Barker played the Reverend in a sketch on The Two Ronnies TV show.
Spoonerisms are prolific in a few other languages. For example, the quirks of the Finnish language (such as vowel harmony) lend themselves well for this purpose, and Finnish 'sanankäännökset' ('word-turnings'), mainly used in jokes, in all likelihood predate Rev. Spooner.
See also: Pun