There are no slots or removeable panels on the iPhone. How else can you possibly access the sim.
The reason it's "pre-installed" and the reason you activate via iTunes is not for convenience or an "innovative way to activate your service." It's because Apple stores are selling them as any other product and are not activating service at the store. That is also why existing AT&T customers have to purchase their phones through AT&T and not Apple stores. You can't transfer your service from your existing phone to the iPhone by simply moving the sim to it. Otherwise, it would work like any other GSM phone that doesn't have a "pre-installed" (aka embedded) sim. [link]
That's the first time I've ever heard of such a dumb idea. Way to go apple :-(
Update 1 July 2007 : the SIM card can be removed, see the YouTube video below: