Generally deactivating dimming
Do you generally want to disable this option? Under System Preferences » Energy Saving, you can disable the auto-dim for battery. Might look different in Lion.
In YouTube, watching videos on full-screen will not trigger the display sleep. I can imagine the same goes for full-screen Skype.
Temporarily deactivating dimming
If you only want to deactivate it temporarily, you can do so by using the Hot Corners feature.
- OS X 10.6: System Preferences » Exposé and Spaces » Exposé » Active Screen Corners
- OS X 10.7: System Preferences » Mission Control » Hot Corners
The trick now is to have a hot corner for deactivating the screen saver – yes, even if it's not running at the moment. Now, when you are using Skype or watching YouTube, just move your cursor in this hot corner. It will prevent the display from sleeping. When you're done watching or using Skype, move the cursor out of the corner again.