I've been seeing pre-orders for this however I can't tell exactly what it is, from the title i can some to 2 assumptions
It is a game that comes before (pre-) the Sequel to Borderlands (ie. Borderlands 1.5)
It is a Sequel set before the titler game (so because it's Borderlands the Pre-Sequel and not Borderlands 2 the Pre-Sequel it is set before Borderlands)
The second point is more reinforced if the Dev's were just being funny by playing on the word Prequel, of cause the Dev's being funny with the name can also explain the first in they wanted a Borderlands 1.5.
So I am wondering, what exactly is Borderlands the Pre-Sequel, is it a Prequel to Borderlands or Borderlands 2