FINAL UPDATE: WWE have posted a brief story on their website, confirming that the set caught fire during preparations for tonight's show. The article states that no WWE Supertars or Executives were injured during the incident, and Raw will go ahead on time at 8/7 Central tonight, 1am UK time.
UPDATE: Since reporting this, I have recieved from multiple sources on Twitter in Cincinatti that the doors opened at 7:30pm Eastern Time (about five minutes ago). Presumably the delay will mean that the Superstars taping (which would usually start around this time) will be pushed back in its entirity to the Smackdown taping tomorrow - although this is just an assumption at this stage. More on this story as we get it.
Chris Jericho has touted a video less than 90 minutes before Raw showing, seemingly, a fire being put out in the arena ahead of tonight's Monday Night Raw.
Jericho touted the video which showed extinguishers being used, and a siren style fire alarm going off in the background, before Jericho screamed down the camera "I'm on FIIIIIRE". You can watch the video below
It has also been said on Twitter, although I would leave this as unconfirmed at the moment, that the building has been evacuated. More on this story as it develops, and what affect, if any, it has on tonight's Raw.