When you are writing some text on the iPhone (any application) and you need to add a punctuation char (comma, semi-colon, full stop etc.) you generall tap the "123" button, tap the punctuation char, and then tap the "ABC" button to switch back to the normal keyboard mode.

There's a quicker way to do it though: press the "123" button and -- while you hold your finger down -- drag your finger on the punctuation char. When you release the finger the punctuation char will be added and the keyboard will still be in normal mode.

To type an accented letter when writing a SMS (or while writing text in any other application, for that matter) press the un-accented letter and hold down your finger until it shows the accented variations; then drag you finger over the accented letter and release it.