After two and a half good years I finally shattered the glass of my trusty iPhone display. Since I had replaced the display on my kid's iPhone 3 before, I decided to do it again to buy time until the iPhone 5 comes.
Everything went smoothly. The online manual has all the details needed. The 3GS is working great again.
The iPhone 3GS is very easy to work on, unlike some or all later models. Fixing the screen may be well worth the $100 cost if you're not ready to upgrade.