5d MK 2 flash underexposure

I've had my mk2 for a week now and was surprised to find that in ettl mode it consistently underexposes shots. I find that I need to compensate +2/3 to +1. I called Canon support about this and the rep told me that it's all a matter of personal preference and that's why the exposure compensation is there. I'm not sure I buy his reasoning because with my 40D I rarely have flash exposure issues. When I take a picture with flash using identical settings, the 5d mk2 will always underexpose compared to the 40D. Other than this small annoyance, the 5D mk2 is an incredible camera. When I first reviewed my first set of pictures taken with it, I was floored at the improvements over my 40D. Btw, I'm using a 580ex II and a 430ex flash.

Same experience here. Are you bouncing or using direct flash? Fwiw, I have almost always used a bit of +FEC with bouced flash. With the 5DII, I seem to always be in the +1/3 to +1 FEC range.

Also the 5DII tends to underexpose in general. I often use +1/3 to +2/3 EC for general shooting.
Of course with flash I will often opt to keep the ambient exposure a bit under exposed.

I've been testing mainly with bounce and shoot through umbrellas.

similar problems reported elsewhere, but thats what FEC is for!