My HTC Desire HD drops WiFi signal quite badly sometimes, I have worked out it is usually when I am holding it in a certain way, the way you hold your phone when you are browsing most web pages which makes it a bit annoying.
I don't know if that means anything though as obviously it's a different phone to yours, but still HTC.
Why on earth put the thing that picks up the signal in an area that EVERYONE will naturally cover with their hands or face when calling / browsing.