AnandTech have posted a review of the Pulse-Eight USB - CEC Adapter here:
It should give a good idea of what could be done with libCEC support
"XBMC is not the only software capable of benefiting from the USB CEC adapter. Pulse-Eight also provides libCEC for download. It is a open source library designed for softwares to communicate with the USB CEC adapter. You can find more information about libCEC here. In this review, we chose not to cover libCEC in detail. libCEC makes a lot of sense for home theater integrators / home automation developers. In fact, any situation involving the need to reliably connect to the correct video source in a HDMI based system can benefit from libCEC. Apart from home entertainment, this includes video conferencing systems and Point of Sale control. But, detailed coverage of these types of applications is beyond the scope of this review. Coming back to HTPC applications, MythTV has already adopted libCEC in their unreleased 0.25 build."
"Support for the USB CEC adapter is currently available to the public only through XBMC Eden. However, Pulse-Eight indicated that Windows Media Center, Event Ghost and MediaPortal would also be able to take advantage of the USB CEC adapter by the end of February along with support for wake from standby through the adapter."
"The bigger picture is the home automation and control scene. It is up to the software community to come up with interesting applications using the adapter, but there is nothing of note in the market right now in that area. There is a lot of scope in the CEC protocol and its potential lies untapped."