I tried to connect to an unnamed wifi network with a HTC Hero and a HTC Desire and they couldn't find the network despite of the fact that I entered all the necessary connection information. Then I revealed the name of the network and the connections were established. After I covered again the name of the network they could still establish connection. So, the solution is to reveal the name of the wifi network for the first connection after that you can hide it again to restore the original state of the network. It works both on Android 1.5 and 2.1 OS.