When you book hotel rooms through Priceline it's clearly stated that you are not eligible to earn for frequent guest points. I've heard second-hand that it's the same for airline tickers and frequent flyer miles.

Shortly after Priceline went public their market capitilization was greater than that of United, Continental, and Northwest combined! Therefore, it's unlikely Delta could have a majority stake in them, though stranger things have happened. For example, 3COM owning 90% of Palm's shares, yet have a market cap half that of Palm's for a time.