It appears the combination doesn't work, using a Composite AV Cable.
Keynote version 1.6 claims to be compatible with the iPhone 3GS with an up to date iOS version. Keynote also claims to be able to display on a TV or projector using a appropriate cable depending on your mobile device capabilities, such as a VGA cable or Component AV cable for the iPad or iPhone 4S. The only option for the iPhone 3GS is an Apple Composite AV Cable (MC748ZM/A).
So far the iPhone 3GS does only sends a video signal for video movies using the Composite AV Cable, not when using Keynote presentations. Did anyone have had success trying to use an iPhone 3GS with Keynote to an external display?