ThinkGeek were selling a Wi-Fi detector T-shirt back in 2007. LatestBuy has just started selling one in Australia.
The shirt runs off four AAA batteries, while the glowing bars will grow and recede depending the strength of 802.11b/g signals (but not n, unfortunately).
The shirt will set you back $50, if you need to broadcast your own wireless signal of geekiness to the world.