Last we heard, Splinter Cell: Blacklist was aiming for a spring 2013 release, though that has since changed. In a dramatic new trailer posted by IGN -- "We're not going to stop this attack. We're going to stop all the attacks." -- a revised August 20, 2013 date was revealed.
Due to the way in which this information was announced, it's not clear what specifically is to blame for the delay. Still, we've heard just about every possible reason at this point, so I'm sure you can use your imagination. This continues to look like something fans of Conviction will enjoy.