The Screen Saver options in OS X Lion are no different from those offered in Snow Leopard, and like Snow Leopard are available to configure in the Desktop & Screen Saver system preferences. As a result, you should expect the screensavers to behave as they did in Snow Leopard; however, a few people are noticing after upgrading to Lion that their screensavers seem to be randomly activating.
If this problem is happening to you, first try adjusting some of your Screen Saver settings by either changing the screensaver, adjusting its activation time, or setting various hot corners that you may have previously used for activating or inactivating the screensaver. If this does not work then the next step would be to remove the screensaver preferences file, in case it is corrupted or otherwise contains settings that are incompatible with Lion.
To remove the Screen Saver preferences, press the Option key and select Library from the Finder's Go menu. In the library, go to the /Preferences/ByHost/ folder and delete the file called "com.apple.screensaver.NUMBER.plist," where NUMBER is a string of alphanumeric characters that will be unique to your computer.
After the preferences have been deleted, go back to your Screen Saver system preferences and configure your screensavers again.