I've got a 400D, 40D and a Speedlite 430EX II.
The flash works just fine with my 400D but not with the 40D.
40D detects it and I can even change settings from my camera (and can see that they change on the flash).
However, it just won't flash. I've tried resetting both the camera and flash to factory settings, removed all C.Fn settings, removed the batteries on the 40D (both batteries).
Even cleaned the hotshoe.
Popupflash works just fine (and won't pop up when external flash is connected)
Any ideas on where to start?