Right now it's possible to back feed power with a powered hub instead of using the micro USB connection. I'd like to see the schematic to find out if that's still possible, but I can't find one yet.
You're bypassing a fuse. If you have a crappy powered hub that isn't well regulated you could fry your pi.
In reality, the chance of that happening is very small, and safety here is only in the context of the "safety" of a $35 board. Most hubs won't let you draw more than a couple of amps anyway.
Well, I'm not sure how well regulated my $3 powered usb hub is. I find myself having to do this weird ritual of plugging stuff in under the correct sequence otherwise I get some power from the hub and the pi won't boot fully (goes into a weird boot loop).
