For some reason, Apple chose to not include a period on the alphabet section of the iPhone's virtual keyboard. This puts a number of unnecessary steps between the end of a sentence and a period, for example.

The New York Times David Pogue has posted a tip to circumvent this problem:

Tap the ".?123" key, which brings up the numbers and symbols section of the keyboard, but hold leave your finger on that key.

When the numbers/symbols section comes up, slide your finger to the "." key and let go.

The alphabet section will automatically reappear.

Tuaw posted an alternative tip in response to Poque's.

Rather than holding the ".?123" key down, tap it like you normally would.

Then tap the "." (or any other character on the number section of the keyboard, for that matter), then hit the spacebar.

As with the previous tip, this will also automatically bring up the alphabet section of the keyboard.