iPhone has an interesting way of handling wifi connectivity. It can discover the presence of wi-fi networks and ask the user which one he or she wants to join. That's great.
Now let's say that you are planning to spend some time in a place with an existing but WEP-protected wi-fi network. And that you have obtained the network's key by email. Can you create an entry on your iPhone with that network's credentials before you connect to the network? No!
The iPhone needs to be physically receiving the networks signal in order for the user to be able to sign in. In order to sign-in you need the network's key. Which could be a hard to remember hexadecimal string. No problem, you think. You could cut and paste it from your email. Not. The iPhone does not support cut and paste. Thus your alternative is to write down the key on a piece of paper!
The iPhone 3G: rediscover the charm of paper notes!