My Iphone 3G had a completely shattered front screen, it was still usable but I wanted a nice clean screen again.
I ordered the Iphone 3G Display assembly, and the install took me like 6 minutes.. two screws out at the bottom of the phone, suction cup the old screen off, remove 3 ribbon cables and done. Putting the thing back together is definitely the hardest part, but only slightly.
Okay for the record the most challenging part of this install revolves around the iPhone 3G FPC Connector which is labeled cable #3 when you open the phone. The first 2 cables which you disconnected labled #1 and #2 just pop off and are easy to reinstall. the FPC connector (cable #3) requires that you gently lift a locking mechanism on the opposite side to where the cable inserts to unlock the ribbon cable before you pull it out or insert it for that matter. You can technically pull the cable out by force without unlocking but you'll never be able to reinsert it without breaking it. So just remember to unlock it first and its a piece of cake.