gee, I thought that the transposition of letters was called a "transposition of lettres".
but seriously, there is a linguistic term that is used for the transposition of two phonemes (letters or sounds) in a word: [b/metathesis. examples of this are seen in the development of the word 'crud' from 'curd' and the (mis)pronunciation of 'pretty' as 'perty'!

see also Anu's own entry on metathesis: http://wordsmith.org/awad/archives/0500