Just updated to iPhone OS 4, but now my wi-fi won't connect. It finds my network and allows me to enter the password but it just says unable to join. Anyone know why or how I can resolve it? Thanks

I have the exact same problem. But strangely mine only won't connect to my own wifi.
I think it is the router.
It connects fine on every other wifi like my office but just not my home.
My solution was to share my internet connection through my iMac.

Mine is an N series router and it had the dual bandwidth option enabled, the 20 / 40 Mhz option. Wouldn't work at all for me in this dual mode.

I altered it to transmit on the 20 Mhz freq instead and it works fine now.

Give it a try, should be an option in the wifi settings on your router.

Did what you said, works fine now! Thanks for that