Get much closer to subjects than usual simply by adding a low-cost macro lens attachment to your iPhone's exisiting camera lens -- with no risk of damage.
Step 1: Get a "pocket" magnifier
Mine was sold as a Jeweller's Loupe. It magnifies 8X. I bought it from an Optometrist. You can also get them in model shops, craft centres and hobby outlets.
Step 2: Cover the iPhone lens with the magnifier
The magnifier must be right up close to the iPhone lens. My iPhone has a silicone case for better grip. I push the magnifier hard up against the hole in it. Then there's no risk of scratching that slippery phone casing or, even worse, dropping everything!
Step 3: Move the camera and magnifier together
Go in close to your subject until you see what you want. Take it. Enjoy!
Here you see the business end of the magnifier and, right in the centre, the iPhone's own little lens. I used a mirror for this shot to prove it really does work.
Step 4: Notes
Work in bright light. The extra optics lose light, so you don't want blurry shots from camera shake. An app like Gorillacam may help with hand-held photos. Mini-tripods are good too but they tie you down a bit with fast moving things like insects.
Make sure the magnifier is centrally positioned over the iPhone's lens, or you will risk cutting off the corners of your pictures. What you see is what you get.
There's no reason why any phone camera should not give similar results.
If you prefer to work single-handed, use Blu-Tak or simillar to fix magnifier temporarily.
Step 5: Distances
frollard suggested I show the kind of working distance this setup gives. As you can see here, it's very close, so take care not to shadow your own shot.
Step 6: Here's the iPhone shot from Step 5
In Step 5 the iPhone 3GS was "told" to focus on the Roman numerals at twelve o'clock (XII).
Step 7: Alternative design of magnifier
This is a lightweight plastic design of magnifier. It is easy to fix to the back of the iPhone using Blu-Tak or similar. This kind is sometimes sold as a watchmakers loupe (lupe). They are also used for inspecting printed circuit boards (PCBs) and woven fabrics like fine cotton or silk.
Now you can work with one hand free.
Step 8: Versatile loupe
This design from Bausch & Lomb (USA) has two independent lenses. Each lens has a different optical power. You can use them individually or combine them for maximum effect.