The most obvious extension right now would be video, and we've already seen it compete with mainstream television in some niche areas. (slaw.ca)
Among others, it appears to be a simple and obvious extension of pre-existing technology. (realtormag.realtor.org)
There is word today that e-book publisher Smashwords is partnering with Sony to make Smashwords - published titles available on Sony's e-book portal (and, by obvious extension, to Sony's line of e-book readers). (ditchwalk.com)