Whilst Neuro has definitively illustrated the actual workings of Canon cameras with flash in Av mode, via the Manual (RTFM) I find it is interesting that this is a fundamental difference between the way Nikon and Canon cameras work.
Canon Av mode with flash works out two exposures, the subject which it illuminates with the flash, and the background that it illuminates with the shutter speed. With no EV compensation set it will attempt to balance the two. ETTL also has a pre-programmed EV compensation, is strong daylight it will underexpose the subject because it assumes you want it on fill duty. In dark situations it will give you very long shutter speeds. Nikon won't do this, the shutter speed will default to a value pre selected in the menu. With Canons if you want to alter the subject illumination you just need to adjust FEC, alternatively to only alter the background use regular exposure compensation. This gives you a huge amount of control to fine tune exposure in auto modes even in dynamic situations where subject distance is changing, or you are bouncing flash. Nikon cameras can also do separate subject and background exposures via the TTL-BL metering mode, but it is not quite the same as the very useful Canon Av mode with flash and their EV compensation also adjusts the flash output.
When shooting Canon and flash in dynamic situations I normally have the camera in M and the flash in ETTL. In less dynamic situations I go manual everything. For macro images, manual everything, the vast majority of macro shooting is as far from dynamic as it is possible to get short of studio still life images.