This tip comes from David Pogue's Missing Manual and is appropriately titled "Pogue's Awesome iPhone Period-Typing Shortcut.
One difficulity in typing on the iPhone is adding proper punctuation as the punctuation keys are on an alternate keyboard, requiring you to press the ".?123" to switch to the puncutation keyboard and then click on "ABC" key to return to the Qwerty keyboard. But no more...
So heres what you can do, all in one motion:
1. Touch the ".?123" key, but dont lift your finger as the punctuation layout appears.
2. Slide your finger a half inch onto the period or comma key, and release.
Incredibly, the ABC layout returns automatically. You've typed a period or a comma with one finger touch instead of three. In fact, you can type ANY of the punctuation symbols the same way.