According to Mike Ozanian of Forbes, the NBA will not allow the Detroit Pistons to be relocated. Although the Forbes story links to a year-old article about the Pistons moving to Pittsburgh, apparently these rumors were popping up again recently with the team still for sale. It seems as though things with Tom Gores are progressing nicely, but until a deal is closed, there will be rumors about the future of the franchise.
I personally never heard any of the rumors about the Pistons moving to Pittsburgh, but the idea of relocation has always been out there should somebody not from the area buy the team. Las Vegas was mentioned once as a possible destination for the Pistons should a non-local buyer end up purchasing the team, but of course nothing ever developed with that rumor.
Forbes' report about how the NBA will now allow the Pistons to move, at least not to Pittsburgh, is based on the fact that the Detroit area could better support a professional basketball team. The Pistons have had many issues in recent years, and Detroit isn't exactly in a great economic situation, but the apparent belief from the NBA is that the long-term future looks more promising for the Pistons in Detroit than another city.
Another reason why relocation of the Pistons is unlikely is because the New Orleans Hornets could be moved once they are sold. Having multiple teams relocating at the same time would be bad for the NBA's image, so that apparently plays into the NBA's reasoning to keep the Pistons in the Detroit area.