Generally, Apple’s 3G iPhone is so reliable that you won’t find much problem with it especially from hardware point of view. But if you are so unlucky in one day when realizing that your iPhone is out of normal operation and best is if you can still send it for warranty claim but if it is not, you may want to figure out the way of how to fix it yourself if you think this is minor problem. However, opening iPhone casing is not always easy as compared to conventional smartphone especially when you are first time experiencing it. Although there are many hardware tools in the market that can help simplify the dissection but nothing is much simpler than the use of suction cup being demonstrated by an online enthusiast.
Instead of plied open it using the hardware tool that could potentially scratch or damage the front LCD panel, this creative way prevents you from in direct contact so that the risk can be held at minimum level. After unscrew the only two tiny screws at the bottom of the casing, apply the suction cup at the surface of LCD, and preferably near to the bottom part closed to Home button area so that correct force can be applied without damaging the flex circuit (there are three flex cables near to the position near to where the internal speaker locates). Then, gently pull the front panel screen away from the back casing and you are done on the first step.
I tried many times and it works great even for a newbie in this field. However, please be reminded that this is only the first step of your attempt to fix it and the rest of steps could be very much dependent of your experience and skill set as well as how complicated the problem is. For details, please refer to video here for step by step guidance and you will be surprised on how fast it can be done without the need of special tool except the normal suction cup that being used for holding your smartphone while driving on the road.