I know this topic is old but I want it up.
I bought 5 WifiShield R3 and 5 arduino uno Rev3.
I got 3 HDG104 chipset WifiShield and 2 HDG 204. I agree with you, with HDG104 it works fine after a firmware upgrade (http://arduino.cc/en/Hacking/WiFiShieldFirmwareUpgrading
) (note that the procedure is not as clean as it seems...)
For the two HDG204 equiped shield, I've the same issue connecting to the simple web server wifi.
One of this card now work fine just repeating many times firmware update (first updating shield and after that updating shield and HD wifi module)
For the second one, it don't work at all and now, the card does not response "Wifi shield not present" on serial monitor.
I wonder if anyone fix that or if arduino team could check that?