My previous favourite geek T-shirt was the 'Will Work for Bandwidth' T-Shirt..... it's now been superceded by this fabulous Wi-Fi Detector Shirt!

 

Why bother searching for Wi-Fi hotspots when your T-Shirt can show you the presence of any 802.11b or 802.11g hotspots...

I'm sure v2 will even show whether the access points are open ;)