Unfortunately it does not work with the 400D but. In principle, you can indeed take pictures and also transferred to the Eye-Fi, but every couple of trips, the camera displays a "Err02 card error" and so it makes no fun, of course. My Eye-Fi has the latest firmware, the 400D also.
I will again one night sleep over it and once again seek nor the search engines. Otherwise, I just, the adapter is returned.
I admit, despite non-functioning two stars, since no one has claimed that exactly this combination should go.