We’ve recently posted a Guide on What You Need to Know About Windows Phone 7 and in that Guide we told you about current limitations as far as we know them today.
Looks like we need to add one more to the list and this might affect you as it surely affects me and I can just hope that the final version hitting the markets in October and November will have this one fixed. Looks like Windows Phone 7, or at least the current build headed out to developers on their dev phones, doesn’t have the ability to connect to hidden WiFi Networks, aka those WiFi networks that do not broadcast their SSID. For added WiFi security, it is a rule of thumb to hide your SSID besides setting a strong password and if you have a set-up following those guidelines, you might need to start broadcasting your SSID in case you want to use Windows Phone 7 on the network, this, again, if the final build will not have this flaw fixed.
(Source: Windows Phone Secrets)