Sasquan, the 2015 Worldcon, started taking hotel bookings today and a friend of mine trying to get into the Doubletree, closest to the convention center, found it is already unavailable. Anecdotal evidence is that the Doubletree filled almost immediately.
Update 09/16/2014: See Kevin Standlee’s comment. It is possible to reserve the Doubletree August 20-22. He suggests doing so, then requesting the Housing Bureau to add the earlier and later nights you want. I have gotten as far as successfully reserving the middle nights online.