Yeah, but not as low power as the DS chip. They're all based on the same architecture, but Tegra 2 is faster and likely bigger than the iPhone, and magnitudes beyond the DS, so it should use up more energy, though it saves a lot of power by using smaller silicon features. The DS design has an advantage compared to phones though -- it has a lot more room for a bigger battery. Nintendo could also opt for a down-clocked chip.
Tegra 2 does make a lot of sense though, and I'd love to see it happen.