Maybe it depends on what is plugged in. As with my PC at work, my classic iPod won't worked plugged into my wife's keyboard with her mouse on her iMac. I haven't tried it on my iMac, which is a year newer and has the aluminum keyboard. There are two elements in how this works - each device uses up a constant voltage, and some products use more power than others.
Does your wife have the aluminum keyboard? And what kind of iMac does she have?