Mac OS X turned seven in March, although perhaps it didn't really start taking over the world until 10.2 or 10.3, in the 2002-2003 timeframe. The "shine" has not faded. Market share is growing. Microsoft tried to "catch up" in the shininess battle with Vista, but they swung and missed. The Linux desktop has a core audience but is conspicuously lacking in "shine" and has so far posed little threat to the Mac's market.
So my best guess is that the iPhone platform has at least five years of life in it, maybe more. Maybe much more -- based on what we've seen with Apple's other products, iPhone has a very good chance of being a dominant platform in its niche for five years or more. It even has a finite chance of becoming the basis of a Microsoft-like multi-decade monopoly.
You can, of course, argue that the hype around the phone will never last that long, just as the hype around Macs and iPods is now over: The "special and new" has become the "standard and expected". That sounds about right. But there's a lot of money to be made in being the dominant vendor of a boring industry standard. Just ask Microsoft.