Sound will normally only come out of one opening on the bottom of the iPhone. One "speaker" is actually a microphone and the other is a speaker.
The original iPhone, iPhone 3G and iPhone 3GS feature a speaker on the left side of the iPhone (with the iPhone screen facing up). The right side is a microphone and produces no sound.
The iPhone 4 has a microphone on the left (with the iPhone screen facing up). The right side is the speaker and produces all of the sound.