Cave Johnson trapped in a computer – as Valve writer Eric Wolpaw revealed during a PAX panel. Wolpaw told the audience that, originally, Chell was going to marry a turret during Portal 2.
Chell was supposed to discover a lost tribe of turrets, help them out and eventually be married to one by the Animal King turret. After this, the turret would follow Chell, according to Game Informer, "in a similar fashion to Watson in Sherlock Holmes: Nemesis."
And that brings us to the sole reason for writing this post: An opportunity to post the Creepy Watson video after the break. For added fun, imagine that Watson is a turret.