Device Identification is a system that has the green and blue person icons, amidst others. This icon tells you if they are even on Mibbit, and if so, what device they are using.
If they have a green icon, they are on Mibbit with a PC or a Mac. If it's blue, they aren't using a Mibbit client (or are not connected to the same server). If it's a picture of a Wii joystick, iPod touch, or an iPhone, they are on that respective device.
See Icons for examples.
There are two preferences under Userlist called Icon Width and Icon Height. That effects the width and height of these icons.
"Show Icons", also under Userlist enables and disables the feature. It is on by default.