Here's a neat trick you can do to type periods and commas and anything else in the alternate keyboard page.
The short of it is: when you want to type something from that alternate keyboard, touch the alternate key but don't lift your finger, instead slide it over to the key you want to insert and then let go.
The alternate key doesn't receive the "key up" message so it flips back to normal key layout, and the key you wanted does receive the "key up" message so it inserts that character.
I'm getting pretty fast with the keyboard. I had a bet with someone who said the iPhone would have a normal keypad in the next iteration and these phones would be sitting on the clearance shelf in six months. I think I'm going to win.