I read recently (I don't remember exactly where) that you could power the Pi from a "cheaper" USB powered hub. I just tried this, and it works.
On checking the Pi schematics, it seems the USB port Vcc pins are directly connected to +5V0, whereas the 5V Power micro-USB goes through the poly fuse.
I don't know what the specifications of USB powered hubs are supposed to be, but if they are just feeding +5V0 to the USB port Vcc (as the Pi itself does) this makes the poly fuse a joke for anyone using a powered hub, and all warnings to people attempting to power their Pi through the P1 port a bit academic.
PS I know similar questions have been asked before. I further tested and the Pi voltage between TP1 TP2 was 4V45, while the 3A PSU supplied 5V25 (off load). I assume this means that the Hub feed is via some circuitry (probably a diode), so if the Pi is using its own PS it should reverse bias the diode, and be "safe". Powering the Pi via the Hub (mine at least) is below the minimum recommended voltage.