HAHA, oh man, where do I start???
Well, it looks like STO isn't getting a THIRD faction after all. Instead, reading the Ask Cryptic, it seems that you can start a new character as a Romulan or Reman, and you get (likely) a handful of new story missions, to bring you into the lore of the species. But after a few levels, you are given the CHOICE of joining the FED faction, or the KDF. This was likely done so that they wouldn't have to create new unique assets for things such as Fleet Starbases (5 tiers) and a new Embassy for Romulans.
In comparison, they went the route of Blizzard in Mists of Pandaria: You create a Pandaren at level 1, play a few levels, and then you choose to be either Alliance or Horde, and that choice is permanent.
Lore-wise, they are basically saying that both the FEDs and the KDF are perfectly fine being allied with Romulans. Call me a lore-whore, but i'm pretty sure most KDF wouldn't be to happy to help, much less be allied with, the Romulans.