Connecting to a non-broadcasting wireless network - Android Forums
The above google result was actually helpful, but took a lot of reading, nerly to the bottom.
What actually worked for me :
You will need the hidden network's SSID and passcode
Go to Android Market, search for Hidden SSID Enabler - it's a free app by Jun Suzuki
Make sure your Wi-Fi is enabled, but not connected to any network
Go to Settings / Wireless and Network / Wi-Fi Settings / Add Wi-Fi network
In the box labeled "Network SSID" type in the hidden network's SSID (name) and click 'save'
Configure the security, if any, enter the passcode as needed, etc.
Now go to the Hidden SSID Enabler program and type the hidden network's SSID into the field given, then click 'Enable Hidden SSID'
That worked for me - after about 15 sec the network connected.