Maintenance crew discovered the body between Bayswater and Notting Hill Gate stations. The BBC now have more on this.
In a weird coincidence, only yesterday Londonist posted a story on urban legends involving corpses on the Tube! Now the police have a real mysterious death to investigate.
UPDATE - According to the Evening Standard, the body was a 38 year old man from West London. The Standard claim he was trying to walk home through a Tube tunnel. The last train had gone through 90 minutes before the body was found at 2.25am. Police examined the last three trains through but did not find any evidence of the collision. Although they have said the victim's injuries, "were consistent with having been struck by a train".