I purchased one of the 1750mAh batteries directly from seidioonline.com and am in the process of returning it. As previous posters have mentioned its capacity is between 1350mAh and 1400mAh. I took my battery to Batteries Plus and they kept it for a few days to test it and reported simular results. I called seidio with this information and explained that if it was a defective battery then they can send a replacement. They pretty much dodged the question and neither confirm nor deny the actual capacity of the battery.
HTC does make an extended 1500mAh OEM battery for the Incredible. It's red just like the stock battery.
Has anyone tried the $5 battery with free shipping from dealextreme?
DealExtreme: $5.01 3.7V 1500mAh Rechargeable Li-Ion Battery for HTC Touch Pro 2
EDIT: on a side note, has anyone else noticed if you run the 1750mAh battery till it dies and shuts itself off and then plug it into wallcharger to charge, it will say its 100% charged after ~60 minutes. Is this some kind of miracle battery? cause I use the same charger with my Touch Pro and it takes ~3 hours to charge fully.