I'm making a powered daughter board to mount my B+ Pi on. Can I power the Pi entirely through the 5v pins on the GPIO header?
More importantly, can I also supply reasonable currents to USB devices through these pins?
Am I right in thinking this has changed between the B and B+?
Yes, you can power the B and the B+ through the 5V and ground pins on the expansion header.
The difference between the B and B+ in this area is that on the B+ you can no longer backpower the Pi through the USB ports.
EDITED TO ADD
You can actually backpower the A+/B+/Pi2 via the USB ports, albeit in a pretty useless way. Once the Pi has been booted via power from the microUSB or expansion header it is then possible to backpower from the USB sockets.
It has always been possible to power a B through the GPIO. An unstated requirement is to ensure the supply is well regulated and not activate the overvoltage/transient protection diode. The recommended practice for HAT is to use an isolation circuit (similar to that on the B+). On the B+ USB can use 600mA/1200MA (depending on settings).
