That'd be all very well and good except that 802.11b runs at 11Mbits/sec and 802.11g runs at 54 Mbits/sec. Those are signalling rates, and there is protocol (and delay) overhead on top of that, but certainly not 60% overhead.
802.11n has some modifications to improve the effective efficiency, so it's better still than that.
802.11n's increased coding gain also means you get better range at the same power, and can therefore reduce the power you're using.
Better still, Nokia phones can reduce their TX power, and yes, you do get better standby time if you do this- more than three days, if you can get away with the minimum setting. You *will* need to set this manually, though.
I wouldn't say it's time to dump wired ethernet- at least, not until we can get gigabit wireless- but things certainly aren't as dire as this article wants to make out.