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. Youve 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.
http://missingmanuals.com/pogues_awesome_iphone_typing_shortcut.csp