Have you ever taken a picture of the sun with your iPhone? You'll notice a little white dot in the lens flare. This little white dot actually represents the face of the sun's disk. It becomes apparent when the sun is eclipsed by the moon, and the little white dot becomes a little crescent sun.
|Left: iPhone photo of the sun not during an eclipse; Right: iPhone sun photo during partial eclipse (I added the arrows)|
No solar filters or fancy tricks needed, just point your iPhone at the sun without any additional equipment or lenses and look for the little white dot in the lens flare. BUT DO NOT LOOK AT THE SUN WITH YOUR EYES! ONLY LOOK AT THE SCREEN! - or better yet, don't look at all... just hold your iPhone up and click a few photos, then see if you got it!
Update: Jon asked a great question in the comments section. So can the sun damage the iPhone camera or image sensor? In short, it's possible since the iPhone camera is mirrorless the light lands directly on the image sensor while you are composing the shot. In my opinion, however, as long as you don't leave the camera in the same position for an extended period of time (longer than say 20 seconds or so) you should be just fine. I've taken lots of pictures of the sun with my iPhone, but try it at your own risk :)
If you are going to go the solar filter or solar viewer route, I prefer a 6"x6" Solar Filter Sheet for Telescopes, Binoculars and Cameras rather than getting a 5-pack of solar glasses. This way you can use it for your DSLR or telescope now or in the future, or cut it up and make twice as many DIY eclipse viewing glasses.
Remember, even through solar filter sheets, the sun will still be extremely tiny with the iPhone camera. Other solutions are a DIY sun projector, or even something as simple as a pinhole solar viewer.