Tehilla L’David (Nun)

This is from wikipedia: According to the Talmud, the letter nun is omitted [from Tehilla L’David] because the verse (beginning with samech) describes God's support for the fallen, and nun begins the Hebrew word נפילה, Israel's future downfall. However, both the Dead Sea Scrolls and the Septuagent contain a verse beginning with the word ne'eman (faithful) ("Ne'eman Hu Elokainu, V'chasid Bchol Ma'asav."), meaning "HASHEM is faithful in his ways and righteous in all his doings"); it is unclear whether this represents an original version or was an interpolation.

from another site: The dead sea scrolls were dated (by use of different and concurrent methods) to date from roughly 150 years before the Churban to about 100 years after the Churban of Bayis Sheini, with the Version that we have today. The consensus is that they are completely different. There are roughly 6,000 differences in tanach between the dead sea scrolls and our Tanach. (The author describes several ‘tracks’ for texts including the Alexandrian, Masoretic, and Samaritain sources which all have differences.)

from another site: The absence of the Nun in Ashrei expresses our sense of loss and incompleteness resulting from the lack of the Temple. Nun is chosen because it is associated with n'fi-lah, Israel's downfall. (Notice that the nun verse should have occurred at the head of the the verse that speaks of God supporting the fallen: Someiech HaShem l'chawl ha-NO-F'LIM).

From http://www.clickonjudaism.org/pages/ethics.html
The Talmud (Berachot 4b) asks about this missing NUN: "Rabbi Yohanan says: Why is there no NUN in the Ashrei prayer? Because [the verse about] the fall of Israel begins with it. For it is written (Amos 5:2): "Fallen (NAFLAH), not to rise again, is Maiden Israel."
Thus, according to Rabbi Yohanan, the NUN is left out of the Ashrei prayer so as not to remind us of a Biblical verse that prophesies the downfall of Israel (God forbid!). However, the verse in Ashrei that follows what would be the NUN verse begins, of course, with our friend SAMECH (it is an alphabetical acrostic after all). And what is that verse? "God SUPPORTS (SOMECH) all the fallen (haNOFLEEM) and raises up all who are bowed down." The NUN verse about falling down is thus implied in the SAMECH verse that assures us that God will support the fallen.

From http://www.hashkafah.com/index.php?/topic/7272-facts/
It seems pretty clear that the perushim at the time of the Qumran community did not have a nun either. Otherwise you would have to posit that in the 200 years or so between Qumran and Rav Yochanan's meimra not only was the nun lost (unlikely in a text that was liturgically important since yavneh) but it was lost so thoroughly that people assumed it never existed. I think it's equally plausible to say that the nun is original and we lost it or that the original text has no nun and the essenes "fixed" it at some point. As a religious descendant of the perushim obviously I'll choose option (a).