Anyone got any idea what type of antenna these Wireless routers that have no "visible" dipole antenna on the outside use?
WiFi frequencies are high enough that useful antennas can be quite small.  It therefore isn't that hard to build the antennas into the case.  Here is a extreme example of a WiFi antenna built right onto the PC board of a WiFi module.  It can be seen near the top of the picture at the top right of the WiFi module.



The whole module from the ends of the solder pads on one side to the ends of the solder pads on the other side is about 1 inch.  The antenna itself is only about 13mm left to right in the picture.  The parts surrounding the WiFi module on the base board are 0805 packages.