(I want to get away from the previous topic!) Parshas Naso contains the formula and mitzvah of birchas kohanim (6:22-27), but at least according to Rashi this is not the first appearance of birchas kohanim in the Torah. In Parshas Shmini, immediately after describing the inaugural avodah of korbanos performed by Aharon, the Torah tells us that Aharon blessed the Jewish people (VaYikra 9:22). Rashi reads that blessing as a reference to birchas kohanim.
Ramban is somewhat dissatisfied with this approach. If Rashi is correct, why seperate the mitzvah and formula of birchas kohanim found in our parsha from the description in Parshas Shmini? Why not place both in one parsha?
I think at least part of the answer lies in the relationship between birchas kohanim and avodah/tefilah. While we never find birchas kohahim recited outside the context of tefilah, it nonetheless is counted as an independent mitzvah. The difficulty of conceiving of b"k as a separate entity is underscored by the question of the Keren Orah (Sotah 38): Given that tefilah may not be a mitzvah, and even if it is a mitzvah to daven once daily, that one prayer can be recited even at night when birchas kohanim may not be said, when and how is the independent mitzvah of birchas kohanim to be fulfilled? The Keren Orah writes an amazing chiddush that the pasuk of "v'nikdashti b'toch Bnei Yisrael" obligates a tzibur to gather for the purpose of sanctifying G-d's name by hearing birchas kohanim!
Perhaps the Torah deliberately created two separate parshiyos of birchas kohanim to reflect this dual identity of the mitzvah: Parshas Naso treats the bracha as a mitzvah in its own right; Parshas Shmini records the bracha as a part of avodah, or as we fulfill it, a part of tefilah.