Care to elaborate? DSPs in many smart-phones / tablets / etc are geared around highly efficient, low power video stream processing - several of TI's newer chips can do 1080p without breaking a sweat for example. Seems like that kind of stream processing architecture would lend itself handily to mining..
CPUs can handle 1080p video without a problem and they're not even optimized for it. DSPs generally don't outperform high-end CPUs, even on the kinds of tasks they're optimized for. They do, however, use much less power, so they're useful in smart phones and tablets. A high-end DSP might do signal processing functions (which mining would qualify as) as fast as a high-end CPU on 1/10th the power. That's still pretty impractical.
I wouldn't expect to get double-digit MH/s out of them or anything, but my understanding was that they were usually glorified FPGAs.
Not really, they're more like glorified CPUs or low-end GPUs.