I've been reading through Gary Knoppers' new book Jews and Samaritans (Oxford, 2013). It's really quite excellent, and I may have more to say about it in a later post. His main theme is that these two religious groups share a close history up to the time that John Hyrcanus destroyed the Samari(t)an temple at Mt. Gerizim in 111-110 BCE. It is now widely accepted that it was this act by Hyrcanus that precipitated the irrevocable split between the two groups, but I suppose it is Knoppers' contribution to emphasize the almost complete lack of evidence for animosity before that point. His chapter on Nehemiah (ch. 6) emphasizes that Nehemiah's opposition to Sanballat was apparently an attitude not shared by many other powerful Judeans at the time.
His seventh chapter treats the Samaritan Pentateuch. This chapter, along with the main emphasis of the entire book, made me think of an as-yet unpublished essay by Jan Joosten which he has already uploaded to his academia.edu page. The essay is called "Septuagint and Samareitikon," and it argues that the Samareitikon was the Samaritan equivalent, not to the Septuagint, but to the later Jewish revisers (Aquila and Theodotion; see pp. 14-15). This means that the Samaritans had the Septuagint and then, like the Jews, revised it toward their Hebrew text, thus producing the Samareitikon. But, Joosten asks, how did the Samaritans get the LXX in the first place? Did they borrow it from the Jews? Joosten suggests the more likely scenario that there were people on the original translation committee who looked to Gerizim as their spiritual homeland (pp. 15-16). Since the major split probably did not occur before the very end of the second century BCE and the LXX was translated in the first half of the third century BCE, it is very possible, even probable, that the translators counted among their members some who looked to Jerusalem and some who looked to Gerizim as the legitimate place of worship.
Now, in this essay Joosten never mentions the Letter of Aristeas, but I am reminded of the composition of the translation committee according to this earliest (and legendary) account: 6 sages from each of the 12 tribes. I had always taken that as sort of a hypothetical, even imaginary reckoning, since, I assumed, the 12 tribes no longer existed after the Assryian destruction of the north in 722 BCE. But in light of Knoppers' emphasis in his book and Joosten's argument in his essay, maybe there is a little more to Aristeas' mentioning of the 12 tribes than I had given him credit for. I'm not at all suggesting that the translation committee really was composed of 6 sages from each of the 12 tribes, but perhaps Aristeas is acknowledging that not only Jews (Judeans) use the LXX and that it was an effort of "all Israel" and not just those in the south. The usual date for the Letter to Aristeas is sometime during the second century BCE, and thus most likely also before the major split occasioned by the Gerizim temple destruction.
UPDATE (15 April 2014): I see that Knoppers has largely anticipated my point here. He notes the pan-Israelite perspective of Aristeas in relation to the identity of the translators (p. 193 n. 65, p. 218 n. 4).