LiPoly battery at 9V makes no sense, the cells are 3.6V or so, 7.2V and 10.8V I could understand.
A bit of research shows that "9V Li Poly PP3" batteries are actually about 7.5V (8.4V max to 6.5V discharged) - if you genuinely want 9V that's fraud in my book. However it looks pretty well matched for powering the Arduino Vin.
Since its Li Poly you need a Li Poly charger (the alternative is risking fire and explosion). Sparkfun have one that has the right configuration I note.
If I owned one of those I would be worried I'd forget one day and charge it as a NiMH.