When using your AirPort Express, it's normal for its case to get warm. The case works as a cooling surface, transferring heat inside the unit to the cooler air outside.
Follow these suggestions to reduce the AirPort Express operating temperature.
Orientation
If you plug your AirPort Express directly into the wall, orient the base station in the proper direction. The cables are designed to protrude from the bottom of the AirPort Express.
Note: You should connect all cables to the AirPort Express prior to plugging it into the wall, except when following the printer troubleshooting technique documented
here.
Avoid insulating material
Do not place your AirPort Express on a pillow or other soft material (carpets, bedding, etc) when it is on, as the material may restrict the flow of air around the base station and cause it to overheat.
General Placement for use with AirTunes
Place your AirPort Express in an open area where there are few obstructions, such as large pieces of furniture. Try to place the base station away from metallic surfaces.
If you place your AirPort Express behind furniture, keep at least an inch of space between your base station and the edge of the furniture.
If you place your AirPort Express in an entertainment center with your stereo equipment, be sure not to completely surround the base station with audio, video, or power cables. Place the base station so the cables are to one side. Maintain as much space as possible between the base station and the cables.
If you place your AirPort Express behind your stereo equipment, place it on one side or the other. Avoid centering it behind your stereo.
AirPort Express complies with the safety standard for Safety of Information Technology Equipment, IEC 60950. This safety standard has been adopted by many countries and is referred to as UL 60950 in the United States, CSA 60950-00 in Canada, EN60950 in Europe, and ASNZ3260 in Australia and New Zealand.