It’ll probably come as no surprise that Adobe is following Apple’s lead & going Intel-only with the next generation of the Creative Suite. That is, CS4 is the last version that’ll run on PowerPC-based Macs. You can read the details in the FAQ on Adobe.com.
By the time the next version of the Suite ships, the very youngest PPC-based Macs will be roughly four years old. They’re still great systems, but if you haven’t upgraded your workstation in four years, you’re probably not in a rush to upgrade your software, either. Bottom line: Time & resources are finite, and with big transitions underway (going 64-bit-native, switching from Carbon to Cocoa), you want Adobe building for the future, not for the past.
