That's news to me.
I haven't run the numbers, but it would seem that using bleed air from the efficient compressor stage of an engine would be much lighter in weight, simpler and much more reliable than adding a several hundred horsepower electric air compressor that the ship has to lug around all its life. Mechanical work still has to be done, so it gets robbed from the engine in some way, no matter what particular system gets used.
The engine is already a very efficient air compressor, so why not use it, rather than increase the load on the spindle shaft and require more shaft energy from the engine to run a larger, heavier generator, to run a several hundred horsepower electric motor, (more added weight) to run another air compressor (still more added weight) for cabin pressurization?
Something tells me that good ol' Boeing may have bitten off more than it can chew with this new 787.
Here's a good read on the reason for the switch to electrical. It's about a third of the way down:
http://www.cocardes.com/articles.php?lng=en&pg=366
Basically, they say pneumatic and (infant) electrical systems are currently equal in efficiency, but pneumatic technologies peaked around a decade ago while electronic stuff has the sky for the limit. Add to that only a few companies are in pneumatics (in a very limited and mature market) while everyone and his brother are working on improving power electronics and electrical devices in the quest for efficiency.
Also, forgot to mention that the bleed air system in an aircraft frequently requires maintenance and the extensive ducting takes up valuable space that could otherwise be used for revenue service. I also wonder about the compatibility of 350 degree bleed air ducts in a composite aircraft.