Just found a great tip for speeding up adding punctuation marks when using an iPhone.
Tap and hold the #+= punctuation button. Without lifting your finger, slide to the punctuation mark you want to enter. Let go and you’re back in the ABC keyboard.
You can do the same thing with the Shift key as well. This definitely is improving my typing speed on the device. I’m not quite at the speed I type on a Blackberry, but I’m getting close. The intelligent auto-correct is pretty accurate at guessing what you were trying to type, and the lack of keys to press means my fingers are not as sore after prolonged data entry.
I was a little concerned that the lack of tactile feedback when typing would be a hindrance, but it’s not as bad as I originally thought. That said, I saw a company that was displaying force feedback displays at SID in May. I bet this feature will be in the followup revision of the iPhone next year, provided that it doesn’t add too much thickness to the device.
tow.com 
Adam,
You can do that with any key on the keyboard. I accidentally discovered this while playing with the iphone last week. I’m not too fond of the keyboard as the keyboard on the Treo is much better.