The email from Apple is very straight-forward:
"With the latest versions of iTunes (9.0.2) and Apple TV software (3.0), you can now use iTunes LP on your Apple TV. We have updated the iTunes LP that was included with [your previous purchase here] so that it can now be viewed on Apple TV in addition to your Mac or PC...."
From there, the message provides a download link. Grab it, sync your Apple TV and you're off. My own is syncing as I type this, so we'll let you know how it looks and behaves later today. In the meantime, enjoy iTunes LP on your Apple TV!