Netduino powerd by USB wall charger
Yep. I'm guessing you have this little gizmo and a random USB extension cable. Looking at the Netduino schematics you can clearly see that the USB input is fused, kinda buffered (can see current limiting op amp and RC PDN arrangement, if I'm reading it correctly), and regulated. The Apple product is supposed to output 5V; I didn't check to see which LDO the Netduino is using, but it'll likely have a drop ~0.3V, however there's a pass transistor as well which may bring that up a bit (maybe ~0.1V drop) -- looks like it may be on the fence between linear and saturation, though, so don't count on it. Surely the Netduino will be fine with a bit less than 5V.