This year I already got five such calls! As well as a couple last year for the previous DevCon.
As far as I know FileMaker, Inc, has not contracted any third party vendors to handle hotel reservations.
Please do not book a hotel reservation with any third parties, and be sure to share this information with your fellow developers.
You should either book the hotel DevCon website for the special rate. And if you are low on money, you can always find a cheaper room, e.g. the Monte Carlo Hotel has rooms for as low as $69, but you have to walk a block to the Cosmopolitan.