"Unable to join the network"

I have an iPhone 3G running 4.2.1. I have never really used the iPhone with Wi-Fi, but lately I tried it out at hotels abroad offering free, unpassworded wi-fi. All of these sites said "Unable to join the network." At hotels that had free, passworded wi-fi, I was usually -- but not always! -- asked for the password but was then told "Unable to join the network." I tried this back in the US, and received the same. I even tried removing the encryption from my own router, and was still told "Unable to join the network." In areas where there are multiple wi-fi networks, I get the list of networks, but I still get the message when I try to connect. What am I doing wrong?