The iPhone polls for email (or periodically triggers Microsoft's ActiveSync with a new listening port - which isn't quite as bad). Polling for email uses data and processor power, thus battery life. With BB, the BB central servers send a flag to the device through the carrier's network via the BB PIN only when there is an updated email / memo / calendar entry / task to be transferred from the user's BES. The BB receives the flag and then gets the data. The rest of the time, no device polling whatsoever. And when the data is finally transferred, it is compressed. Talking about a direct comparison between the two devices for email usage, a network could support several blackberrys on the same bandwidth used by one iPhone.

Incidentally, the routing of that flag from BB's central servers to the individual device without the device needing to maintain a TCP connection is why there is a different data plan required to support BES / BIS. BIS users do received "polled" email, but the BIS server does the polling and only flags the device to receive when there is a reason to.

Here is a comparison of Blackberry to ActiveSync. The difference between the two is drastic enough, but remember that device based polling (ie POP or IMAP on an iPhone or Windows Mobile device) would create FAR more usage.