In a recent interview with IncGamers, Star Trek Online Lead Designer Al Rivera revealed that the Borg (among other races) are being considered as playable races for the game.
When asked what other factions (besides Federation and Klingon) players could expect in the future, Rivera answered, "There is no shortage of ideas from the IP. Certainly, we have considered Romulan, Star Trek Online Borg, Dominion, and more." Not willing to offer any concrete confirmation, however, Rivera cautions, "We are not committed to anything specific at this time - we're just focusing on Federation and Klingons right now."
I imagine anything is better than a flat out "No" for eager Borg fans out there.
For the full interview with Al Rivera, head over to IncGamers.