I book travel through a corporate travel website, (AMEX is behind the site). And their rates, for selected hilton properties, are substantially below what I ever see on hilton.com/hiltonhhonors.com (typically $40 to 50 per nite difference). The tradeoff is that not all rooms in the hotel are available at the lower rate, coworkers and I have discovered that when the corporate travel website says no rooms are available, it really means no rooms are available at the negotiated rate, but if you call the hotel directly, there are rooms available at the 'rack rate', but not at the corporate rate.

I also see, when booking via hilton.com, at the 'Dates & Preferences' page, there is a field for 'corporate account #'. I'm curious if anyone uses this, how you found out your corporate account number -- my employer takes pains to get all travel booked via the corporate site -- and if you get/got the same rate as booking via your corporate travel website?