Apple equipped iPhone to tap into a proprietary infrastructure, not unlike the one RIM uses for BlackBerry, that pushes mail and PDA updates to iPhone over the air. "Push" is a relative term that's entirely dependent on your software.
iPhone being a consumer device, it typically will be used with low-end mail and scheduling clients that hit the server at timed intervals, or over a hotel or conference center wireless LAN could be using an SMTP proxy that delays delivery. It takes as long as 15 minutes for iTunes to pull an update or message from Outlook or Entourage to Apple's cloud, at which point it finds you within a few seconds. But in an enterprise using Exchange Server and iPhone, push can be taken for granted. Pushing desktop-sized messages is best handled with the iPhone 3G.
Most of what iPhone can do over the cellular network, it can do over Wi-Fi. You can run both networks simultaneously on an iPhone, and the iPod Touch with the iPhone 2.0 firmware makes a great on-campus PDA (too bad that microphones like the Griffin iTalk won't work on it; that'd give you some voice capability when on Wi-Fi networks).
The iPhone 3G is world-compatible, supporting four varieties of GSM and three flavors of UMTS. If you have a contract with a carrier that supports roaming, you can now hop on a plane and expect your iPhone 3G to connect for you when you land.
One drawback with iPhone's Exchange Server support is that each device only supports one user profile. Multiple users wouldn't be sharing one iPhone, but one user might set up different profiles for the various projects he or she is working on. It is possible to add multiple non-Exchange profiles pointing to POP or IMAP servers; I used that technique to work around the single profile issue by creating a secondary IMAP profile for my Exchange Server.
iPhone 3G additions
E-mail with rich attachments that used to be impractical over the AT&T EDGE network are now workable with the iPhone 3G -- provided you're within range of a UMTS tower and your coverage plan includes HSDPA high-speed data. In my metro-area tests, downloads reached as high as around 1 Mbps, and rarely dropped below 700 Kbps. The iPhone 3G will automatically fall back to EDGE when 3G isn't available. You don't lose data access. It just slows down, and if HSDPA comes back into view, it speeds up.
There are catches here: Consumers who expect to stream low-res movies and listen to streaming radio all day will bump into AT&T contract provisions that put limits on unlimited data plans. Apple had to rebrand iTunes for iPhone to a more explicit "iTunes for Wi-Fi," making it plain that AT&T has no interest in having Apple send you movie rentals over its cell network.
For professional users, the sticky wicket is cellular data's horrible latency. During my speed tests, I measured 3G network latency at between 270 and an astonishing 1,100 milliseconds. You'll notice that some pages render faster than others, and that Web sites with lots of little AJAX image buttons can load slowly the first time. Apple's marketing of iPhone as a cellular browser should be taken with a bit of salt. Test Web applications carefully before deployment, and be sure to test the iPhone 3G in an area served only by EDGE. (If you're in EDGE territory, you may be better off with an original iPhone upgraded to the 2.0 software, since you'll pay $10 less per month to AT&T.)