I first read about this tip from waitingforiphone.com which leads you a page at O’Reilly’s website. Here’s the skinny. The keyboard layout on the iPhone is great for the most part, unless you want to get to the numbers and punctuation marks via the “.?123” key in the lower left-hand corner. Normally this would take 3 taps to get to a period.
Here’s how to do it in one single motion.
Tap the “.?123” key but keep your finger pressed down. As soon as you do this, the punctuation keyboard layout will appear. Now (with your finger still pressed down!) slide your finger to the desired key and then release. Upon release, the desired character will be inserted into your text and the normal alpha keyboard layout will re-appear so you can continue typing alpha characters.
This is an excellent tip and will definitely speed up typing and texting for everyone.