Here are some tips for using the iPhone's built in speakerphone.
If you want to begin using the iPhone's speakerphone mode start by selecting Phone from the lower task bar, which is what you select to make phone calls in general. Once you have begun to engage in a phone call you will have a number of different options brought up in front of you, such as viewing the keypad again or putting it on mute. The option in the upper right corner of this box will be speakerphone. You just press this option to activate the iPhone speakerphone function, which will highlight it while it is in use. Once you are done using it during the course of a phone call you can just press it again to turn the speakerphone off.
If is still pretty easy to do other iPhone functions while using the speakerphone. If you want to go online with Safari while you are talking to someone with the speakerphone function you can just start by hitting the Home button. This will keep the call active, but take you to the regular iPhone desktop. Here you can select Safari and activate it while still talking. This will not work with all applications, and especially not most applications that are not built into the iPhone.
iPhone 3Gs Speakerphone
The new iPhone 3Gs includes an automatic speakerphone function that the iPhone 3G did not. What the iPhone 3Gs does is go into speakerphone mode if you are talking with the phone at a long distance from your face. This is estimated using the iPhone 3Gs's on board infrared technology. This can cause problems for iPhone 3Gs users when it does not work correctly, but often times this is because of a carrying case that they are using with their iPhone 3Gs. You have no way to turn off speakerphone permanently so that it does not come on.
Feel the Power
The iPhone's speakerphone is surprisingly powerful for a cell phone. For the most part you can keep it at a fairly good distance from you and remain talking without much sound loss. This is perfect for having the iPhone nearby for discussion, but moving around a confined area. You do not have complete freedom of motion and should still remain within a few feet.