In Tsakonian (a modern descendant of Doric Greek), the letter combination ΣΧ is pronounced [ʃ] (the first sound in English "ship").
However, it seems clear that this wasn't the ancient pronunciation, since this combination wasn't used to transcribe Hebrew [ʃ] in the Septuagint.
When did this pronunciation start? Or is it a purely orthographic device, not applying to words that naturally have sigma and chi together like σχίσμα (schisma)?
Since no-one has provided a more definitive answer, I will provide what I think is probably the case.
Tsakonian, being considered a dialect of standard Greek, is unlikely to have any type of traditional orthography, so any spelling conventions are comparatively recent. Therefore, the use of ΣΧ for [ʃ] is most likely a simple orthographic device, unrelated to historical development. I will guess that its use was suggested by the fact that in standard Greek ΣΧ is pronounced, at least before a front vowel, as [sç], which is acoustically similar to [ʃ]. Quite possibly, the German use of "sch" for the same sound provided additional impetus for the choice.
Not definitive, I know, but I think something along the above lines is highly likely.
The Wikipedia article on Tsakonian gives some information on relevant sound changes. This supports varro's conjecture that the Tsakonian spelling of ΣΧ for [ʃ] is merely orthographic, and doesn't reflect a sound change like [sx] > [ʃ].
Relevant points from the Wiki article:
