I was very excited that google street view was announced the same time I bought my iPhone, alas I could not figure out how to use it.
Apple’s or Google’s websites offered no help, so it took some google searches to reveal how to do this (comment 3 here).
Basically, you have to drop a pin and tap it. If street view is available at that location, there will be a small red circle with a white person on the left hand side of the pin’s label. This also works with pins from search results.
Tap that icon and the screen will rotate into street view. A small circle indicates the street location and cone of view. It doubles as the exit from street view, which is cute, but not very intuitive.
Unfortunately, there appears to be no way of knowing where street view is available as there is in google maps on a computer. All you can do is drop a pin, tap it, and see if street view is offered. This can be misleading if you are at the border of a street view covered area, where just a small distance over it may or may not work.