This is a sample shot with no flash exposure compensation. I had the main dial set on manual - 1/80s at f/5.6. No ambient light is coming through, this is all bounce flash. Note the histogram has nothing in the top 1/4, where the most information is stored (in terms of digital 'bits').
At first I thought it was because when the flash is bounced, there isn't enough light, but when I set the flash to manual on full power (1/1), the scene ends up almost totally white. This tells me I have plenty of power, even when bouncing.
Here is a second shot with the exposure compensation for flash turned up to +1 1/3. This completely changes the histogram to look 'right'. Data all the way, and more what you would expect in a scene like this. Note that I'm not claiming this is exposed properly, but nothing is blown out, and you can pull back a bit on exposure after the fact. In the previous case, if I pull the exposure up, I get a lot of noise in the shadows with my 20D.
I find this to be the case with my Canon flash when used in ETTL. I almost always need overexposure on the flash to get a properly exposed shot. I typically run with +2/3 when shooting in auto.
The bottom line is that you need to go manual if you want to nail the exposure every time, as ETTL shifts around a bit depending on the brightness and reflectance of the subject.