(pic provided by Jenny since I don't have nVidia 8)
The awesome Jenny Thielt wrote instructions on how to enable dynamic shadows on the Second Life 1.23 Release Viewer (re-posted here with permission):
Enabling Dynamic Shadows on the 1.23 Release Viewer and any viewer based on the 1.23 version.
Make sure you have a DirectX 10 compatible card (eg. nVidia 8 series or newer)
1. Make sure Atmospheric Shaders is on (windlight pretty skies and clouds)
2. Open the Advanced menu (if you dont have it yet press ctrl+alt+d)
3. Go to Debug Settings…
4. type renderuseFBO
5. Change value to TRUE
6. type renderdeferred
7. change value to true
8. Welcome to the world of dynamic shadows.
To disable dynamic shadows all you need to do is change the renderdeferred value back to false. (you can also change the renderuseFBO value to false too if you want)