I recently had to do a presentation on Grid computing and Clusters, and according to the data and bits of information I was able to scrape together, Bitcoin Pools should be considered to be Grids.
Computer Cluster: A group of linked computers, working together closely thus in many respects forming a single computer. The components of a cluster are commonly, but not always, connected to each other through fast local area networks.
Grid Computing: The combination of computer resources from multiple administrative domains to reach a common goal. grids tend to be more loosely coupled, heterogeneous, and geographically dispersed.
You're correct, OP
Its a Grid.