I cannot confirm that - I have booked a number of times using my customers' corporate rates (including the HP and IBM ones mentioned), and was always rewarded the points - I guess it depends on the individual hotel...

I normally call Hilton Reservations directly and ask for a specific corporate rate. Sometimes they even tell me the corporate code on the phone, but even if not, the account number will show up in the reservation summary later (at the bottom).

Hope that helps,
pilluelo