I have seen a Making-Of filmed in the Square Pictures HQ on Hawaii (IIRC). They had an impressive PC cluster, I wouldn't say it was 1000, but it could be, which they used as a render farm for finished scenes. The main work was done over SGI Domain Render Servers, and these beasts have abilities which you just can't achieve with standard clusters. Else, if a 1000 PC cluster would be enough to render such a movie, any company, Pixar, Disney, they would just buy HUGE PC clusters. They would be cheaper than a huge render server, but they don't since they need those features, you just don't get bandwiths of up to 700 GB/sec with PCs, but if you take a look at how many people worked at the same time in this office, creating and rendering the scenes, you will realize how big the traffic to these servers is.
PC-Cluster, as well as a Mac cluster, could be used, and sometimes are used, but they just don't reach the performance of a "true" render server.