...if you want to do any sort of computational intensive work like video encoding on a portable device, you must resort to the power of the graphic processor, and the only way to fully extract processing power from a graphics processor is to split work into multiple pieces.
Yeah, you're describing what a GPU does, not what a modern DSP architecture necessarily does.
Sophisticated DSPs run the same DSP encoding/decoding algorithms as CPUs, but are essentially RISC chips optimized to run only these operations and leave out unneeded functionality (like floating point operations, for example) so as to be extraordinarily fast at running those specific sorts of algorithms.
Although there are multi-core DSPs (like the Cell processor), modern DSPs don't use the hundreds or thousands of simple cores that GPUs do. They might have 8 or 16 cores, but parallel process data in a manner much more like modern CPUs do. In fact, clusters of DSPs are being effectively combined as transputers to serve as RISC CPUs, as in the case of the aforementioned Cell processors.
Anyway, it's fascinating stuff if you want to take the time to read up on the technology.