You can see the content of your screen, but there is broken glass that makes it hard to see. Cracks, splinters, and shattered glass can occur when dropped, or upon a high PSI impact to the iPhone 3GS. While not often, sometimes screen will break inside pockets, usually due to pressure from keys or other hard objects impacting the screen. The Digitizer (what reads the touch of your fingers) will typically still be responsive, but can also fail, if not immedaitely, over time.
Preventative Measures include using a case for your iPhone that creates seperation between objects and the screen. cases that don't create a 'lip' on the glass side of the screen may be fashionable, but provide no protection. An Otter Box™ protective case on the other hand, provides significant protection against impact damage to the front, back or sides of the iPhone. Screen protectors provide limited protection as they do not prevent PSI breaks (Pounds per Square Inch) where pinpoint pressure causes the screen to crack, however, it will aid in protection against sliding cracks where impact is minimal but objects harshly abrase the screen.
iPhone stops responding to touchscreen commands. This can occur with impact damage (usually in association with cracks in the glass for the screen), extreme heat (such as laying in the sun for extended periods of time) or water damage to the device (although sometimes the mainboard is too damaged to use a new digitizer/glass replacement). *Note: most items including gloves, pens, or other objects will not work to repond to 'touch'. Preventative Measures include using an Otter Box™ protective case, and keeping the iPhone out of the sun for long periods of time.
Aesthetic repair for those with scratches on the external glass. Glass can scratch from a variety of things, including (but not limited to) keys in the same pocket as the iPhone, skidding across rough surfaces (concrete, walls, even hardwood floors). Preventative measures include a plastic adhesive clear/ anti-glare or reflective screen protector, a ZAGG Invisible Shield ™, or a case with a protective layer such as the Otter Box™.
There are two layers of glass for the Digitizer Assembly, the first is simply glass, which is what you touch. The second is attached to the first, and senses the touch via an electrical field that transmits and recieves based on the 'return' from your finger or stylus. The advantage of the iPhone Digitizer, is that it runs independantly from the LCD, so even with a dead LCD you can still use the iPhone. TO read more, check out Ejizzle4550's article on how the iPhone Digitizer works
No. They are the same size, the look the same, but they are not. A 3GS Digitizer will not plug into a 3G iPhone, or work to any degree. A 3G Digitizer will plug into a 3GS, but you will immediately notice that where you touch or swipe is not where the iPhone thinks you did. While they look similar, understand that they are NOT interchangable. View the iPhone 3G Digitizer
Yes. It comes with a 1 Year Warranty against manufacturing defects or installation errors, including the part and labor to replace it. Please ensure that you pick up a warranty card at the time of repair. The warranty explicitly does not include any screen that is cracked, shattered or broken in any way, damaged due to fluid or water, or if the iPhone was repaired by any other company. Warranty validation may require further inquiry than initial inspection.