iPhone 3GS screen repair
I went through this myself a couple of weeks ago. Stupid tile floor in my office wouldn't catch my phone at 9.81m/s2. I ended up taking my phone into a mobile repair shop and had them replace the screen (voiding my warranty in the process), but was a hell of a lot cheaper than doing it myself. Took all of about 15 minutes.

Your profile doesn't list where you live, but any Apple Authorized repair shop can do it for you (or send it away for repair if they can't do it in house).

That said, it is possible to do the repair yourself. iPhone 3GS glass replacement kits can easily be purchased on eBay and from any other number of resellers. The process for doing it yourself isn't easy, but depending on your own level of handiness, you could do it yourself.

There are some excellent video guides for DIY iPhone glass replacement on YouTube. For example: http://www.youtube.com/watch?v=4mboB8p-sdw