The mute button in my iPhone 3G broke and, therefore, I was unable to move it away from "mute" and I was missing a lot of calls. Using a needle, I managed to put it back to "ring", but then it went off very inconvenently at meetings.
Although the button is a small thing, the job is tough, as almost the whole iPhone has to be disassembled to reach it and replace it. Luckily the iFixit guide was very detailed and the "big" version of the pictures really helpful.
I printed some of the key disassembly pictures and used them to leave the screws I was removing in their position as in the picture. This enabled me to reassemble everything without getting confused as to which screw goes where (there are many along the way).