The New York Times is reporting that Hewlett Packard spied on its directors to find the source of some leaks. The paper reports that this included monitoring home and cellular phones, and that the company's counsel had advised that it was lawful. The article is a bit short of details (did they wiretap the phones or buy the calling records from brokers), but I can't imagine actually listening to these calls would be lawful. See: H.P. Spied on Directors to Find Leak - New York Times.
Update (20060906): Wired is reporting that the investigators bought calling records. See: Wired News: Phone Scam Charge Rocks HP.