Marketcircle just released an app called iPhoney, which allows developers to simulate the iPhone browsing experience.

From the product page:

iPhoney requires Mac OS X 10.4.7 or later. Here’s some what you can do with iPhoney:

Test your iPhone-enabled Web 2.0 applications and compatible web sites.Open any website that works with Safari.

Rotate to see websites in either portrait or landscape orientation

Show or hide the location bar for a full-screen iPhone experience.

Simulate the iPhone user agent, to test browser redirection scripts

Marketcircle plans to make iPhoney open source in the near future.

When using it to test iPhone apps, keep in mind the following issues:

• The iPhone user agent isn’t toggled on by default, go to iPhoney->Use iPhone user agent
• Scrollbars appear in the simulation, which don’t appear on the actual iphone, so factor in those extra 20 pixels.
• It appears larger than an actual iPhone on screen due to the differing resolution scaling (160ppi on the iPhone), so if in doubt, scale your text 150% bigger than is comfortably legible in the simulation.