apparently you're not alone I'm having the same problems and so are numerous people from updating to 3.0 the best solution in the apple forums is to have your iphone replaced check the thread here
No, i dont have 200$ for a refurbished iphone OMG, screw apple, they messed up my iphone, why should i have to pay for their software problems? omg, this sux
yeah restart the phone, .
then general settings, scroll down to the reset option. erase and reset all network settings. same thing happened to my 3gs and my 2g with 3.0... the wifi is not picked up only after the reset of the network settings