I think for many of us it will simply be a timing issue.
I'm perfectly happy with my iMac G5 Rev. b, and I see no compelling reason to even be considering a new system yet. A few years from now, things will be different. Streaming HD, Blu-Ray or HD-DVD, 20+ Megapixel cameras, etc... eventually all computers get old. When the time comes, apps will be x86 native, and dual / quad cores will be the norm.
If I needed a new computer now to do my tasks, I'd be getting one now. But I don't, so the switch looks to be at least 2 years for now.