As of Precipitate 1.0.3, you can now change the default application that opens your "cloud" documents.

You simply need to change the hidden pref CustomLaunchers:

defaults write com.googlecode.precipitate CustomLaunchers -dict-add "com.google.precipitate.GoogleDocsSource" "com.apple.safari";

To use an app other than Safari, (i.e. a FluidInstance) use the Fluid bundle identifier:

defaults write com.googlecode.precipitate CustomLaunchers -dict-add "com.google.precipitate.GoogleDocsSource" "com.fluidapp.FluidInstance.Google Docs";

Bonus: Here's a tip to add Precipitate documents to Quicksilver.