First, enable the Develop menu (as indicated in the earlier hint; it used to be called the Debug menu). Then, from the new Develop menu, select User Agent » Mobile Safari x.x.x - iPhone. Try it on Google, for example, and you'll get the right-sized iPhone Google pages. Cool.
[robg adds: Most browsers include some method of setting the user agent, so you can view iPhone-only sites in pretty much any browser you want to use.]

