Dear "Brianwar",
I shouldn't worry about a listed nominal voltage of 3.7. I've just checked the standard Fuji NP-50 that came with the X10, and although it has been charged but unused for weeks, it shows 4.13!
I bought a pair of aftermarket batteries from this source last December:
http://www.ebay.co.uk/itm/2X-New-Ba...91?pt=UK_Camera_Batteries&hash=item35bdf38553
One was a dud, but replaced without quibble, and without the need to return it. The batteries seemed to take a long time to 'run in', but appear to hold charge better than the original, although not in proportion to the claimed 1500 mAh I think.
At a neighbour's wedding last week, I took a minimum of 273 shots with the X10 without needing to replace the aftermarket battery. I've no idea if this is good - I've never before taken so many shots in a day with any camera!
Oh, and by the way, I've just recharged that battery, and it shows 4.14V.
Hope this might help.
Pete