Ever tried to get a Windows Mobile device on to a really secure Wireless network? I'm not talking about your home network here, I'm talking about a network secured with a Radius server, PEAP, certificates, 802.1X and TKIP.
Its a less than pleasant experience. First you manually have to install the certificates, which means exporting them from the CA to files and then getting them onto the device, easy enough if you're a system admin... less easy for Joe User. Once the certificate is installed you can them enable Wifi and login to confirm authentication. And then randomly have to re-login to the same network depending on what you have connected to in the meantime.
And on the iPhone? Turn on wireless, enter your login details, accept the certificate and then just use it as necessary.
Just a bit easier...