iPhone simulator for Windows
Just to add additional information to this post:
found another one for both iphone and ipad: http://code.google.com/p/ibbdemo2/downloads/detail?name=iBBDemo2.air&can=4&q=
Google Chrome now has the ability to "Toggle Device Mode" by clicking the Phone icon in Dev Tools. This gives you a more touch-specific interface than just using the browser, allows you to throttle data, etc...
From here, you can choose the device:
You can do it all online, without a simulator:
Keep in mind, it's not a real test on an iPhone, but if you use it with Safari or Chrome for Windows, you'll come really close to how it'll look on an iPhone.
UPDATE: iphonetester.com no longer exists.