Wow! I had a look at the source code and managed to figure out how they did it... They are using embedded iframes with a specific height, width which crops the Google Ads icon....
In other words the Google ad is on another html page which is embedded on the display page which crops the ad icon out so that it is not displayed... I hope that makes sense... Its not obvious to see but when you follow the external javascript and css file you can then see how they put that together with printing out the div holder..
I'm pretty sure that it is against Google Adsense TOS lol... That said I can see why they would do it ;-)... It actually has got me thinking, whether I should try something like that myself !!!
Curious to know if anyone can confirm if this is against Google Adsense TOS?