Reading mashable last night and being a new iPhone 4 owner I found one of their latest posts most useful!

Using your iPhone keypad while typing tap and hold the '.?123' button, then slide your finger to the required key and let go! Instant update without having to tap to switch screens!

I found this incredibly useful with punctuation, tapping away for my message then sliding my finger for a comma, job done!

* So much so this has all been typed from my iphone :)

mashable.com