Dear Mark, 

<Carsten> is a northern German name and conforms more to Low Saxon phonology
than to Standard German phonology. Traditional Low Saxon does not shift the
clusters /st/ and /sp/ to [St] and [Sp]. The Low Saxon word for "stone" is
<steen> [stEIn] whereas Standard German <Stein> is [Staen].

Yours, Dan


-----Original Message-----
From: Mark Reed

"Sorry to belabor a point that has so little bearing on your actual
question.  But if it was at one time Car-sten, what prevented the [s] from
becoming [S] in accordance with the usual initial-cluster phonology?"