Both vendors I spoke with, eCoupled and PowerMat, said that the charging coils are broken into two categories. One is lower power for devices that require less than 100 watts (cell phones, MP3 players, etc.), and the other is for higher-power devices that require more than 100 watts (laptops).
If you place a smaller device on the higher-power charger, it won't work, because the system is smart enough to recognize that it would damage the device.
The charging coils are mostly built into mats, on which the electronic devices are placed. It also can be built into other surfaces, such as desks, tabletops, or countertops. This would allow you to charge devices by simply placing them on your desk. This video below shows how it all works: