My boss is confident that Blackberry phones are safe for company use (having it connect to the email server, etc.). He is less confident about iPhones. Regarding connecting to a corporate email server where security is a huge priority, is there anything different about the iPhone which would make it more of a risk.
SnapOverflow
The iPhone utilizes standard SMTP/POP3, which neither are secure protocols. If your SMTP/POP3 servers support SSL, you can enable it on the iPhone. It'll secure content between your device and server.
Blackberry's unique advantage is, when coupled with a Blackberry Enterprise Server, can be completely secure by tunneling internal emails to a mail server on firewalled network.
But, no matter what you use, if sending to an Internet email address, it leaves the secure network and therefore insecure by default.
BlackBerry devices rely on mail transiting RIM's network operations center (NOC). Because the mail is encrypted by the sending endpoint (either the BES server or whatever they now call the desktop client) the risk is relatively low. However, it's not zero. The bigger risk with RIM's setup is probably downtime, not interception or tampering of messages in transit.
As spoulson points out, the iPhone answer depends on whether you're using IMAP/POP or Exchange ActiveSync (EAS). EAS devices, like the iPhone, connect directly to the Exchange server using SSL. That means that all inbound and outbound traffic is SSL-secured, so it's quite safe.