Flash
Both the Sigma DP1 and the Canon G9 have very small, weak flashes. Both added some useful fill in sunlit shots, but neither was a strong performer as a main light source indoors. The cameras accept dedicated flashes made for the DSLRs marketed by their manufacturers. We're not familiar with the Sigma flash, but we have used the Canon 580EX, which is a flexible and capable unit. The flash units are larger than the cameras we're testing, so it's not clear that anyone would make a habit of using them together. Still, accessory units would be the way to get useful flash results with either the G9 or the DP1.