As Craig said, it's important to do this because a) "After only eight months, iPhones now account for a remarkable 71% of all US Mobile Browser web traffic;" and b) designing for the iPhone requires "much more than simply designing for a smaller screen size."
The Apple-authored simulation program is apparently buried deeply within the iPhone SDK (software development kit), which is free to download. Though you need to sign up for the free Apple Developer program first, you don't actually need to develop Apple software. They won't make you take a programming test, in other words. ;-)
iPhone Simulator story
You can read the full tip and SDK download instructions here:
Now, what's "QuickTips"? It's a weekly HTML e-mail with a couple short tech tips, often accompanied by video, primarily for creative professionals. If the tip goes longer than a paragraph or two, a link brings you to the full tip on their web site, such as the iPhone one above.
QuickTips is written by the staff of Creative Techs, a Macintosh training and consulting company that serves customers in Seattle. I'm one of thousands of non-Seattle groupies who subscribe to their weekly e-mails … sign up, it's worth it! Just enter your e-mail address in the subscribe form in the iPhone story above.