Zero squint factor
For professionals, the PDA features of a handset may be the decider. iPhone's calendar and address book are absolute tops for ease of use, fast access, and readability. The use of spinning slot-machine-like dials to set the time of an appointment is more intuitive than arrows, and it's quicker than typing. Appointments can be separated out by category (for example, "personal" and "office") or pulled together into a single view, in which case the category is reflected in the color of the entry.
Apple makes great use of the tall display and Mac OS X's crisp text rendering. All of the calendar views use the largest type that will fit while still packing as much information onto a single page as possible.
Contacts are listed in a massive bold font. A new search field at the top of the list displays only matching entries, and for rapid scrolling through a large list, there is an alphabetical index tab down the right edge of the screen. Each contact can be assigned its own ring tone and avatar, and you can add custom fields to each entry.
Calendar lacks a key ability: to send calendar invitations via e-mail. But anyone who has permission to do it can place an event on your calendar, and when they do, your iPhone will be updated over the air. If you're linked to Exchange Server, new appointments are sent to you via ActiveSync push, and you can accept or reject an invitation. But you can't send one from iPhone. That's a major flaw.
In the plus category, big time, is a new rich attachment viewer that displays Office, iWork '08 (Apple's productivity suite), PDF, and many flavors of still images inside the Mail app. Using pinch (shrink), spread (zoom), and flick (scroll) gestures, documents that are too large to fit on the display are easier to navigate than on most other devices, and iPhone is surprisingly quick with the document conversion.
A colleague pointed out the shortcoming that non-image attachments can't be saved, sent, or transferred (except in forwarded messages). I'll add that there is no way to create rich documents on iPhone, no equivalent to the mobile office suites on Nokia E-series and Windows Mobile devices. But I expect to see third-party document editors appearing on App Store, Apple's on-line custom iPhone software catalog, soon.
iPhone is a wireless device. You should look at its USB cord as being for charging, backups, firmware updates, and iPod content. For professionals and enterprises, wires will just get in the way.
The Contacts and Calendar apps can sync over the air to several servers including Exchange Server, Apple's MobileMe, Google, and Yahoo. iPhone will sync, through iTunes, to Mac and Windows desktops (Outlook or Outlook Express for Windows, iCal for Mac), but users report mixed experiences with this. I'm not surprised. Doing tethered sync with a device that's optimized to do it over the air -- that's a major win in iPhone 2.0 -- is counterproductive. Sync efforts tend to pull up a lot of false conflicts that must be sorted out manually. It's better to take new events, contacts, and messages as they are posted. The servers that dispatch them are more reliable sources than your desktop.
Apple's MobileMe is billed as "Exchange Server for the rest of us." That's a bit rich, but it does keep multiple iPhone, iPod, and Mac clients in sync, and the AJAX front-ends to the Mail, Calendar, and Address Book are slick. MobileMe syncs Safari browser bookmarks as well. (I haven't tested MobileMe's sync features against Windows.) I wouldn't make MobileMe my sole e-mail server for business use, but I think that the service, which costs US$99 per year, is a necessity for iPhone users.