While Microsoft's Windows Phone 7 supports WiFi by default, it's part of the hardware specs every device have to follow, the Windows Phone 7 UI is rather simplistic and only shows WiFi networks which are broadcasting the SSID (WiFi network name). As a matter of fact it means that Windows Phone 7 devices are unable to connect to networks which are hidden and not broadcasting their SSID - for security reasons. However, HTC has just released a tool called "Hidden Wi-Fi" which allows users to add 3 WiFi access points with hidden networks to the device list of WiFi networks. After the hidden network is added, it can be connected like any other WiFi network.
Hidden network is now available free of charge from the Windows Phone Marketplace but exclusively available for HTC Windows Phone 7 customers.
Cheers ~ Arne