you have to account for ALL consequences and all expenses involved before you can say that it will save money.
Sure, we can save a little electricity here and there in the server farms and it's safe to assume smart data centers that rent gear will already be buying more efficient gear to save on their own electricity costs since it cuts into their bottom line.
Don't forget that energy hungry CPUs also incur additional costs such as the A/C to keep it cool, more UPS gear needed to support the servers during a power outage and then there's the impact on real expensive gear like backup generators.
Just the fact that I'm using a shared resource in a large server farm instead of trying to self-host or colo my own gear is GREEN IT in the first place as it shares the same A/C, UPS gear and backup generators as thousands of other servers, making it a savings purely from an economy of scale POV.
Therefore, since I rent a server with such low monthly hosting prices, they have to be keeping all their expenses under control, including electricity, otherwise they'd go out of business and/or the price per server rental would increase significantly.
Not that saving electricity with smarter CPUs and more energy efficient boards isn't a good idea, but there are usually larger economies of saving energy to be done in any inefficient organization.
For instance, forget servers for a second, those energy efficient netbooks are more than sufficient for many workers in an office that don't need the power of a desktop and a big 24" LCD monitor.
A receptionist could just as easily type letters and send those "WHILE YOU WERE OUT" phone call notifications using a netbook than a power hungry desktop and it's more than sufficient to play solitaire between phone calls :)