Here's the quick summary of my experience:
- Helicon Focus Pro Multiprocessor does use multiple cores/processors, but with diminishing returns. Their web page quotes 1.7 times faster with 2 cores, but only 2.3 times faster with 4.
- TuFuse uses only one.
- CombineZM uses only one.
- ImageJ uses multiple cores/processors for many built-in functions, but the registration and stacking plugins from BIG use only one (and those plugins are agonizingly slow in addition).
Done really well, there are huge gains to be had. One research group in the stitched panorama community has been developing a new stitching code that is specialized to use the SIMD instructions of the IBM/Sony/Toshiba Cell-Processor. They report that it runs twice as fast on a Playstation 3, compared to the fastest available competing code running on an 8-processor AMD PC. The cost-effectiveness is even more striking, bearing in mind that a properly configured Playstation 3 costs only around $450.
There is, however, one huge difficulty: the software has to be redesigned from the ground up to get this sort of performance improvement. Most likely it'll happen, but just when is anybody's guess.