I mean that even if the AC-8E is labeled "5V", its real output is usually more than 6v, which will be too high for many USB devices.

All nokia powersupplies with 2mm plug are allowed to vary between 5.2 and 10V, and all the ones I have are at around 6V.

Edit: But if you're planning to create a external power supply to charge your N8x0, please don't just look at "10V" above and think you can feed it directly. At 5.2V the powersupply must have a current limit of about 1000mA, iirc, and at 10V this is closer to a 100mA limit...