I too jumped too soon and purchased one of those Hong Kong replacement batteries before reading these threads. It was much lighter than the real one so therefore does not have as big of a battery inside of the casing. It won't even power the N810 for more than a few seconds. Luckily I only waisted $6 USD on it.

So anyone have sources for the best deal on a real 1500mAh BP-4L?

If the best price is close to the Mugen one listed above and if the Mugen really does have 300Ma more power (I'm not sure I believe it unless they designed their package so that it had less outside casing and more battery inside. Can somebody with both weigh the two to see if the Mugen battery is heavier?)

My next question will be for a 12V Automotive supply that also has enough power to run the N810 as many can only power smaller Nokia phones.