Interesting replies. I still believe that Xbox Live has sold 360s, although at a slow and steady rate that is difficult to recognize. You want another example? The Wii Remote. Now that is something I believe we can all agree has sold systems. Or how about the size of the Wii? Or the wireless controllers for all systems?
What these all have in common is that they are abstract and their appeal is long lasting -- they don't get people to jump out of bed and rush to get a Wii/360/PS3 the day these services are released, but they influence decisions people make throughout the generation. They aren't games, more of a change of philosophy or system design for which the rewards are difficult to calculate.
Which explains, again, why we can never really prove or disprove whether these things actually have much effect. Big games we can see; the system in question sells 40k more systems than it otherwise would in the month the game is released, and then has no visible effect ever again. Something like the Wii Remote is affecting people even today, influencing parents, for example, who want their kids to play something more active. And again, I expect the PS3 to sell 500-1000 more systems per month (worldwide) throughout its lifespan as a result of home; for reference, that's maybe a .2 to .4 percent increase per month, but I expect that increase to last forever, as Home is permanently integrated in to "what it means to own a PS3 instead of a 360 or Wii."