Digital media is where the commercial computing market is heading, it is where most of the action will be. Apple has a core group of customers, who have been with Apple a long time, who use Macs primarily for digital photos, sound and film. Cell is optimized for digital media and computational work; this makes it very attractive to people who work with digital media. You can connect the dots from here.
Regarding the Cell processor's roots in the Power5 architecture, perhaps it is best to quote an article in the IBM Journal of Research and Development, authored by the Cell architects. You can find this article at:
Three sentences stand out:
1. "Indications were that a completely new architecture can easily require ten years to develop, especially if one includes the time required for software development. Hence, the Power Architecture* was used as the basis for Cell."
2. "By the end of 2000 an architectural concept had been agreed on that combined the 64-bit Power Architecture*  with memory flow control and “synergistic” processors in order to provide the required computational density and power efficiency."
3. "The Broadband Processor Architecture extends the 64-bit Power Architecture with cooperative offload processors (“synergistic processors”), with the direct memory access (DMA) and synchronization mechanisms to communicate with them (“memory flow control”), and with enhancements for real-time management."
Regarding the difficulty of extracting Cell processor performance from a programming perspective, a large effort has been underway to provide developers with the necessary programming tools. Intel has been doing the same thing for their Core Duo architecture. A muti-core, multi-processing environment is about to become the new norm; those who refuse to adapt will suffer the same fate as others in the past who didn't change with the technology. If you want to develop relatively simple applications, then you will likely be able to continue using your current programming model. If you program on top of engines such as Apache, then you also won't likely need to worry about it; the Apache developers will port to the new environment and you won't have to deal with it.