With 560 airwires still unrouted, I'm clearly not done with the ALU board. But there's enough to see how the long vertical signal traces break up the power plane, yet leave wide top-to-bottom paths to supply VDD.
At least you'll be able to see it if you make the image large enough! The original is 2560 x 1569 pixels (shrunk to 1600 x 980 by BlogSpot) so there should be plenty of detail.
If this was a truly high-speed logic board with fast rise times, the current would have to detour around the the long vertical breaks in the plane rather than being able to follow the same path as the horizontal signal traces (not shown here). To work around this I'd need very low impedance bypass capacitors between VDD and GND to allow the current to jump to the unbroken ground plane. Or so sayeth the book -- I don't have the personal experience to be able to vouch for it.
I'm actually making pretty good progress on this board, though at this rate it'll be at least another month before I have the ALU and ID boards ready to send off to PCB-Pool for fabrication.