By focusing on Intel Macs, we save a huge amount of engineering and testing time. This means that we will be able to complete more features for a larger group of customers and deliver the best release possible. Plus, some CS4 technology is so new that it never existed on PowerPC Macs.
Adobe believes that by removing support for the PowerPC architecture they will be able to deliver more features in CS4. For users requiring PowerPC compatibility, Adobe recommends buying After Effects CS3 while it is available.
While other Adobe applications are also Intel only such as Premiere Pro, Encore, and Soundbooth, the decision shouldn't reflect on other Adobe applications such as Photoshop (which has seen its share of CS4 controversy).