We chatted briefly with Pete Hines today to get a follow-up to yesterday's story about Fallout 3 DRM, but there's a more complete statement on the Bethesda Blog:

For Fallout 3’s copy protection on PC, we use the same security model as we did for Oblivion - a simple disc check. We only use SecuRom’s disc check functionality for copy protection. We do NOT limit the number of installs. We do NOT use online authentication or any other SecuROM functionality except for a disc check when you install the game and when you launch the game. We do not install any other programs and we don’t have anything that runs in the background while you’re playing the game.

If you experience issues installing the game, try running setup.exe directly off the disc. That will resolve most compatibility issues.

Get the latest information from our tech support forums, including this specific thread related to SecuROM.