Sony? They were already producing components for the living room, of course, but the Playstation (and the Xbox) has become nearly what everyone imagined STBs to be in the 90's.
If I remember correctly what a set-top box was supposed to deliver, its functionality was restricted to small, limited tasks: give you access to satellite/cable TV programming. It would sit next to your TV set, plugged into the AV port.
Limited functionality, because back then more sophisticated features would have cost more money.
The typical customer for set-top boxes was not necessarily the end-user. For example, hotels would buy them (the customer), and the guest (the end-user) would use them. As a developer you could do well selling your set-top boxes to those customers, if the price was right, and even if to the end-user the product really sucked.
The PS3, XBOX360, AppleTV etc. don't really fall into the set-top box category of old because the customer is the end-user, and the devices are much more powerful. Also, the medium which the set-top box allows you to access is no longer necessarily analogue, but digital. Things have changed so much, probably for the better.