Language and writing system

Nick, I think that principle may apply in some orthographies, but it certainly isn't a general one. So, for example, ö is a letter in some alphabets and a diacritic in others.

There was a related discussion about the Dutch IJ on the ATypI list recently, and one of the things I noted was that there seems a tendency for digraphs that cannot be confused in pronunciation (e.g. English VV, Dutch IJ) to become letters (W, IJ), while digraphs that can be confused in pronunciation (English CH, TH) do not become letters.

[The box and oval diagram above is specific to Indic scripts, and shows basic types of consonant and vowel information relationships. Not shown is consonant+vowel ligation that happens in some scripts. But this is independent of the question of what constitutes a letter or not in an alphabetic script.]