Using cap locks on iPhone or iPod Touch devices is a useful feature that most people never use. When twittering I use this all the time… LOL, BFF, ABITHIWTITB, XOXOXO, etc…
For some reason with the iPhone, to type a series of upper case letters most people do the following: press shift key -> let go and press the letter, press shift key -> let go and press letter -> rinse, repeat, etc.
However, just like a real keyboard, you can just hold the shift key while you press the keys you want to be upper case:
press and HOLD shift key -> press letter, press letter, etc.
Alternatively, you can enable double tab shift lock:
Settings -> General -> Keyboard -> Enable Caps Lock
Once this is enabled, tapping the shift key twice will turn on caps lock. When the caps lock is enabled, it will glow blue.