"Spoonerism"- from dictionary.com

spoonerism \SPOO-nuh-riz-uhm\, noun: The transposition of usually initial sounds in a pair of words.

Some examples: * We all know what it is to have a half-warmed fish ["half-formed wish"] inside us. * The Lord is a shoving leopard ["loving shepherd"]. * It is kisstomary to cuss ["customary to kiss"] the bride. * Is the bean dizzy ["dean busy"]? * When the boys come back from France, we'll have the hags flung out ["flags hung out"]! * Let me sew you to your sheet ["show you to your seat"].

Spoonerism comes from the name of the Rev. William Archibald Spooner (1844-1930), a kindly but nervous Anglican clergyman and educationalist. All the above examples were committed by (or attributed to) him.