This is not an available feature in a direct way. Since D* bought ReplayTV, I'm hoping they may make it available in the future though.
You can use a Video capture card to record the content, but you are stuck at a real-time download. Think of it as copying two VHS cassettes. For better picture quality, look into purchasing a HD capture card that handles Component Video (Red, Green, Blue cables for video) as this will offer the best resolution available.
Search the forum as there was a recent discussion on this.