Ralf Herrmann’s Wayfinding & Typography has several interesting articles concerning the infamous German letter ß, aka sharp s or eszett. What’s a Ligature, Anyway? explains the difference between typographic and orthographic ligatures, and why ß in contemporary Latin script is neither. Rather, it was defined in 1903 as a single letter in the new German standard orthography. (The earlier history of ß is a different matter, though. To my knowledge it began in Old German handwriting as z with a tail, and later received a long-s prefix in blackletter to distinguish it from an ordinary blackletter z.)
The 1903 decision had one problem: there was no uppercase ß! Unlike blackletter, Latin script is quite frequently capitalized, and that left typesetters with a dilemma. Using lowercase ß looks just plain ugly, whereas the German Army transcription SZ looks weird because it doesn’t match pronunciation. The common transcription SS is even worse: German words and names frequently contain an actual ss, making the capitalization ambiguous, and double consonants shorten the preceding vowel whereas ß does not.
In 2008, Unicode 5.1 finally resolved this long-standing problem and introduced Capital Letter Sharp S (U+1E9E) to allow for proper reversible capitalization of Latin Small Letter Sharp S (U+00DF). Font support remains spotty – Georgia still lacks the new glyph, which is why I can’t type it here – but manufacturer Cherry is already putting Capital Sharp S on keyboards, so I expect it’s here to stay. Interestingly, this orthographic change seems to have been driven by technology: human clerks and typesetters could handle the traditional ambiguity, whereas computer algorithms require an unequivocal representation.