what is your concept of "user"?
I'm no "power plant expert", but I worked for several years at a power production company, so I have some clue what I'm talking about.
First of, you are right in that a power consumption reduction of a single W will not make any difference at all in total emissions, as you can't adjust production to that degree of precision. Additionally, most power plants always run at full capacity, as running them on partial capacity is not nearly as efficient. However,when the total demand on the regional grid drops several MW, entire power plants will shut down to save fuel (in my case the "regional" grid covers Sweden, Norway, Finland and Denmark, approximately 25 million people, so that only a few W per person). Whichever power plant on the regional grid has the lowest fuel efficiency ($ per produced MW) will shut down first when consumption drops, and start last when it increases. On this grid, and probably on most grids, those are the Oil Plants (daytime) or Coal Plants (nighttime, when the Oil Plants are turned off anyway due to lower demand), and shutting down one of those makes a huge difference in emissions, even per W produced (on average).
I'd be more interested in knowing if MeeGo finally ships with a working Poulsbo driver!!?