It may be an issue with the metering system, or your apeture is stopped down to much. Try using a wider apeture and higher ISO setting such as 400. You should also read
FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE - Canon Digital Photography Forums yes, it's from a Canon site, but it does have a lot of good information on indoor flash photography. I feel your pain with the K10d. I sold mine two months ago in favor of a Canon after coming back from Mystic with 200+ dark pictures that were shot in bright sunlight.