Just made a booking for IC Sao Paulo using the "guarantee". The lowest rate excluding all the taxes was $530 on the IHG's site.
Third party site was selling the same room for $504 including all the taxes besides some city tax. The price was lower anyway, but the best rate guarantee lady calculated the competing rate down to $480 when excluding the tax.
The phone call took about 20 minutes. Got a case number. She told me that they are contacting the hotel for them to adjust the price down.
I just have a feeling that this guarantee for a completely free nights is going to be short lived.