I'm fairly sure this is an Asimov short story, but I can't actually find the name of it. A group of kids has a robot that tells stories. They are mean to it, and generally abuse the thing. At the end of the story, the robot tells a line to itself in a different voice, something about computers in general becoming intelligent and getting revenge. Sorry for the lack of details, but it's been a really long time since I read it.
You are right, it is an Asimov story called "Someday".
The computer is called the Bard and the kids dislike it because it's not as fancy as the sweet new Visual Bard.
It ends with the Bard saying "and the little computer knew then that computers would always grow wiser and more powerful until someday...."