As you probably know, the iPhone attempts to auto-correct as you type. If you key in "blogget," for example, it'll automatically replace it with "blogger" when you hit the space bar. But did you know the iPhone can compensate for missing punctuation as well?
Try it. Type "can't" without the apostrophe (and then hit Space). Presto: the iPhone fills it in for you. Same goes for words like "don't," "isn't," and "you're."
What this means is you can tap along without switching to the number/punctuation keyboard every time you need an apostrophe.
If you need another kind of punctuation, try this: tap and hold the number/punctuation button, slide your finger to the mark you want, and then release. The iPhone will insert the symbol and return you to the QWERTY keyboard.
Of course, there's another typing time-saver most people already know: a double-tap of the space bar adds a period (and space) at the end of a sentence. And you probably know this one as well: you can tap and hold some letters and punctuation marks to bring up a menu of alternate letters and/or symbols.