from roaming around a bit, it seems that there is an explicit federal law covering pre-texting when it comes to financial data (Gramm-Leach-Bliley Act). but, when it comes to telephone records, there seems to be some debate on if current statues make it illegal and there's discussion on passing laws to make it explict. since they went after telephone records and not financial data, Gramm-Leach-Bliley Act does not apply. another thing to note is that Gramm-Leach-Bliley Act makes it illegal to ask someone else to fraudulently obtain financial data on your behalf. so, my take away is that since there is not a corresponding statue that relates to telephone records, even if it was illegal for the hired investigators to pre-text, it probably was not illegal for HP to ask the investigators to obtain the information.
while there seems to be some question as to the legality of HP's behaviour (assuming it was only pre-texting and not HP divulging SSN's), i don't see that there is much of a question on how ethical this was.