Why does my WLAN stop working when I plug in my HDTV?
Bit of a shot in the dark but have you tried a different cable?  Maybe the one you're using is damaged and is shorting something somewhere which is affecting the wireless antenna.  It is definitely a weird problem by the sounds of it but you could try a couple of tests by first using a different cable, then using a different TV with each cable and see if they all produce the same result.  If they do then it might be worth getting Apple to take a look at your DisplayPort socket if you're under warranty or a good repair shop.  I've seen weird things happen to computers with dodgy USB cables so there's a chance it's something like that.