Anyone who reads Cicero's letters cannot fail to notice that he quite frequently uses nos and noster to mean ego and meus. Earlier I heard a paper where nos in Lucretius' proem was meant singularly (primarily because Venus in Epicurean thought cannot have actually helped Lucretius like the Muses help epic poets).
This made me wonder where this plural actually comes from. Is it just a stylistic development? Is its use in English as the pluralis maiestatis derived from Latin? Wikie also suggests its use in other languages, but they're usually fairly traceable developments; Andreas Katsouris 1977 ("Plural in place of singular," RhM 120: 228–240; he suggests that the use of it in Roman comedy may have come from Greek translations, but provides nothing further than that) examined the phenonmenon in Greek, but what of Latin? Is it related at all to the poetic plural?
In case you're not entirely sure about its usage, see Bradley's Lessons in Latin Prose p. 212 (with cleaned up commas):
Obs. 1. Nos, noster, or a plural verb are often used when only one person is spoken of in preference to ego, meus, or a singular verb. This idiom is rare in narrative and in historical writers. Instances of it abound in Cicero, especially in those parts where he is writing in a philosophic, conversational, or epistolary style. Multis de rebus scripsimus, I have written on many subjects.