This would be a first for DC, which has four shows based on comic book characters on TV right now. Thus far, the only shows sharing a universe have been the two sharing a network. But according to Greg Berlanti, that could be about to change.
In the November 5th print issue of Entertainment Weekly, there's a profile on co-creator and executive producer of Arrow and The Flash, Greg Berlanti. Tucked into a description of a day in the life of Berlanti is this quote:
Later in the afternoon at his other office on the Warner Bros. lot, Berlanti meets with fellow producer and Ali Adler and and Oscar-winning costume designer Colleen Atwood to go over looks for their upcoming revamp of Supergirl. This show will follow 24-year-old Kara Zor-El, Superman's cousin, and in a twist that could melt fanboy brains, Berlanti says it's possible that this Supergirl could enter the worlds of Arrow and The Flash.
DC's been reticent to cross network boundaries with their shows — which makes a lot of sense for Gotham, whose time-bending approach to style makes it hard to place in the clearly modern contexts of the other shows. Of course, with Berlanti being in charge of all three, it's likely that Supergirl will have a similar tone to the CW shows, which'll make integrating them a snap.
Berlanti also gave Entertaiment Weekly a quote about the pilot which indicates that season one of the show isn't going to be a traditional origin story:
The events of the pilot get her to finally emerge as the true hero she was meant to be.
Which raises an important question: Does "finally" mean she's been around for a long time before the show starts? Are we talking the usual "stops a crime" plot as the impetus for getting her to be hero?