It would seem pretty secure but the data is still traveling via wireless so it can still be intercepted. There are several tools available that will allow people with a little computer knowledge to see a wireless network even with SSID turned off, grab the wireless data as it travels then decrypt it and although they may not be on your physical network they could pull enough data to find your WEP or WPA key along with the MAC address and Network settings which they could then use to join your network.
Although this would probably be highly unlikely it is a possibility and the tools to do this are open source so they are available to anyone and require very little skill to use. Here is just a quick list of some of the more common tools that can be used:
http://www.oxid.it/cain.html
http://www.kismetwireless.net/
http://airsnort.shmoo.com/
http://stumbler.net/
One thing you can do is download the tools and use them to test your current network to help improve your wireless network security. Or another suggestion would be to consult with a security expert.
Hope this helps,
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