I can't do a full discharge test yet as I don't yet have a LiPoly protection circuit built and don't like objects that explode. But I'm using both the 100mAh and 860mAh batteries from Sparkfun.
I wouldn't mess with trying to tie the charging into the Gumstix/Audiostix USB, to be honest. At the very least, you'd have to cut the traces to the onboard regulator. And you'd have no protection/switching circuitry.