If you're setting up a vintage Snow Leopard Mac and want iWork '09 running on it, there's a gotcha: you'll need a serial key.

This defaults write bypasses the trial:

defaults write /Library/Preferences/com.apple.iWork09.Installer InstallMode Retail

And then to suppress the registration prompts that pop up every time you open Pages, Keynote, or Numbers:

defaults write /Library/Preferences/com.apple.iWork09 ShouldNotSendRegistration -bool yes

That's it.