With two extra batteries, you'll be able to shoot all day on a single charge.
Shooters with the Sigma SD1 rejoice, as your DSLR of choice will be getting the PG-31 battery grips in the near future. Newly announced, the accessory holds two dedicated BP-21 battery packs, effectively tripling the amount of time you can spend out shooting. As with most battery grips, it also has a dedicated shooting button to aid vertical photography. With a weight of 10.6 oz (300g), it'll add some serious bulk to the camera, but that's not unexpected.