If you've owned an iPhone or iPod Touch, you'll know exactly what I mean. The default settings are sometimes annoying and confusing. I've round up a few tips for you:
1. Caps Lock - You can enable CAPS LOCK under Settings>General>Keyboard>Enable Caps Lock
2. SMS Reminder (iPhone only) - Say you received a message, read it but did not acknowledge it by interacting with your phone. It will remind you twice more since it assumed you did not see it. This can get annoying very quickly. To disable it, go to Settings>Messages>Repeat Alert
3. .com Options: When typing in a URL in Mobile Safari, hold down the ".com" button and you'll find three other alternatives: .net, .edu and .org. Subsequently, when typing in an email address, hold down the period "." button and the same four options will be available as well. Generally speaking, holding down period "." will let you toggle between one vs three periods (. vs ...)
4. iPod Audio Setting: To adjust settings for your iPod, ironically you'll find it within Settings>iPod (instead of within iPod itself). This is where you can change EQ, volume limit, and other important functions for video/tv playback.
5. Screenshot: Hold down the home+power button at the same time. It will snag a screen capture and store it in your photo album.
6. Battery Reserve in % (iPhone 3GS only): Settings>General>Usage
7. App Store Sign Out: This can be found within Settings>Store
One thing that bugs me the most is the inability for iPod (while playing video) to adjust for screen brightness; to save battery, I normally keep the brightness setting at around 15%; very annoying to have to quit video just to adjust for brightness in Settings>Brightness.