The difference between home use and corporate use.
It's usually possible to get any device to link to any mail system. But in a corporate environment the fact you can do something is outweighed by the cost of implementing and supporting it.
Most corporations have gone for Blackberrys because they do everything the business needs them to do while at the same time being easy to set up and maintain. For them to take on another device would require investment in the systems to support the devices, staff training in using and supporting the phones and increased implementation time for future corporate systems to ensure compatibility. So far the iPhone doesn't offer the corporation anything to make that investment worthwhile
For the iPhone to break into the corporate market it would need to out Blackberry the Blackberry.