rboren : I'm sure you're already aware of this, but I'm still gonna say it : the main problem is not with the number of letters used in the abbreviations, but the fact that two days are duplicates of two other days, which makes the calendar unusable to other language.
Other language also use the same letter for some days, but not the same day as english. Here's how it should display (as best as I could understand - let me know if I made a mistake):
uk : S M T T F W S (1st day of week is sunday)
fr : L M M J V S D (1st day of week is monday/lundi)
pt : D S T Q Q S S (1st = sunday/domingo)
it : L M M G V S D (1st = monday/lunedÃ¬)
de : M D M D F S S (1st = monday/montag)
...which is still understandable (for these languages, I understand others DO need more than one letter).
The first problem to solve (and I guess it already is, or is in the works - again, I'm not trying to tell you what should be done rboren, as I'm sure you're already aware of these issues)... erm, the first blablabla solve is the "first day of the week" issue. Combine this problem with what other languages see in WP and you get this kind of calendar (again, let me know if I made a mistake) :
uk : S M T W T F S
fr : D L M M M V D
pt : D S T Q T S D
it : D L M M M V D
de : S M D M D F S
Hard to tell what's what. If you'd ask me : too many M & S :)
I'm writing this as some kind of reference, hopefully that will be helpful to someone... I'm going to add a Known Bugs section in fr.po, so that no one gets a surprise...