Spoonerisms

"Spoonerisms" are accidental exchanges of letter sounds or syllables within words, phrases, or sentences: "it is kisstomary to cuss the bride," or "flutterby" for "butterfly." They are not uncommon and, like the "tip of the tongue" phenomenon have been studied for some time now for what they tell us about language organization and processing in our brains.

see more spoonerisms here

Wikipedia entry here

The satirical comedy troupe, the Capitol Steps, have become famous for their 'spooneristic' (taken to extreme) comedy bits over the years; audio example of one here:

http://www.capsteps.com/sounds/lirty-parishilton.mp3

...and more of their offerings here.