And for what it is worth - the clients will disclose the SSID's they're searching for - hidden or not, and with smartphones/tablets/laptops, it's every SSID they've ever attached to, along with the MAC address of the AP...
So, someone sitting in a coffee shop, with the right tools, they'll know where you've been, based on what your WiFi client is asking for - hidden or not...
sfx
And once they know what SSID you want to connect to, it's trivial to blindly accept you, and then start capturing your POP3 passwords as your email client tries to connect to the server (and a vast majority of users do not use SSL/TLS on their POP3 accounts).
That's one big reason why I recommend people NOT to hide their SSID, as they will then turn their client into a security liability. I know Microsoft fixed something related to that after WinXP, but how torough their fix is, I don't know...