Update 01/2008: SIP-based VoIP on the iPhone has been coming:
When Steve Jobs first demoed the iPhone in January 2007, he made it clear that reaching someone by typing their phone number onto a keypad was no-longer acceptable, albeit tolerated. Instead, he showed an Address Book interface that unifies the concept of a "Person" across all forms of communications on the iPhone, be it iChat, e-Mail, or a Normal Phone Call. Since version 10.2, Mac OS X has pushed Address Book integration across disparate Applications to unify the concept of a "Person", and iPhone simply builds upon the same philosophy.
While an "Address Book" seems as trivially simple a concept as it isn't new to anyone who's used a mobile phone within the last decade, seeing it executed "The Apple Way" in a larger synchronized ecosystem, helps paint a picture of possibilities that lie ahead.
Picture unlimited free calls over WiFi/IP without even having to "think about it", by simply picking a Person from your Address Book, and hitting "call" ... The same way you'd make a Normal Phone Call. All this powered behind-the-scenes by an outstandingly executed convergence of enabling technologies and open-protocols.
E-Mail addresses define a universal mechanism for a message to make its way into our INBOX, regardless of who provides the sender's E-Mail service. E-Mail functions on-top of open and interoperable standards. As such, there is vibrant competition to obtain "E-Mail Service".
Wouldn't it be nice to enjoy the same type of competitive and interoperable landscape when it comes to actually speaking to and video-conferencing with people? Beyond those silly phone numbers controlled by a handful of phone companies, what if we could pick from a myriad of competing "Providers" to obtain a global address allowing people to "call us" over the Internet, regardless of who their "Providers" are?
This is where SIP comes-in. It's been around and used on computers for a while, but it's been waiting for the right combination of enabling devices and software to truly break onto the handheld mainstream.
iPhone, with its WiFi capability, Address Book integration, and advanced operating system, is getting us one step closer.
A SIP Address looks just like an E-Mail address. A Person's SIP Address could easily be stored in the iPhone's Address Book. Apple could build SIP-capability right into the operating system, pre-configured with a number of existing SIP Providers for one-click setup, while still allowing for custom configuration, following a model very similar to E-Mail.
There are a few SIP Providers out there. But Apple could easily roll out its own SIP infrastructure as part of the .Mac framework, increasing their chances of providing a superior out-of-the-box experience, while promoting the .Mac brand to ... competitive usefulness. From here, the sky's the limit as to what Apple can do, leveraging iPhone's brand and near ubiquitous and still increasing WiFi penetration. Forget about fighting over 3G vs GSM. WiFi and IP are universal world-wide.
SIP call quality can be vastly superior -- think CD quality -- to a Normal Phone Call, as it strives to remain pure data exchange over the Internet Protocol on broadband connectivity, without ever getting wedged thru the codec limitations of any Normal Phone System.
When calling somebody, the iPhone could detect whether WiFi connectivity is available, and whether there is a SIP Address for the person i'm looking to call. If both these conditions are met, the iPhone could perform a "pure SIP Call" over the Internet, without ever touching the carrier's or any phone company's network. Blam. Free call. An icon might indicate to me that this call is a free, un-metered Voice-over-IP call.
Even if the person i'm trying to call doesn't have a SIP address, some SIP Providers offer the ability to relay calls to the Normal Phone System (aka PSTN in industry jargon), at substantial cost savings for unlimited plans, at which point all the iPhone has to care about, is that WiFi connectivity is present.
Update 6/21/2007: As pointed out in comments below, WiFi+VoIP on "phones" aren't new, and carriers have somehow "dealt with it". Even if Apple further blurs the line between VoIP and Normal Phone Calls, there are still profit incentives and competitive market pressures that may very well entice AT&T to embrace VoIP.