Ha, I had this same question not long ago. Just when you think you've got it, Canon makes you think you don't. The on-camera flash will still flash to communicate with the slave, but it does it before the shutter fires.
To make it work, set the off camera to slave, channel 1, group B. On camera to master, channel 1, ratio off. It will work, you'll only be tricked into thinking it doesn't.
Not sure about the 550ex, but suspect it may be similar to 580ex where the main flash is disabled via the zoom button.
The zoom button controls four different functions on the 580ex in Master / ETTL mode:
1) Zoom
2) Ratio on/off, A:B, A:B C
3) Channel 1-4
4) Main flash on (light rays blink) / off (lighting bolt blinks)
Note that even with the main flash disabled the Master will still pre-flash to instruct the slave. That occurs between the time the shutter is fully pressed and the shutter is opened so pre-flash and main flash are difficult to differentiate from behind the camera. But if you see flash through the viewfinder, its pre-flash so don't sweat it.
Also worth noting, the Master always defaults to group A. If just shooting with one slave you'd want to leave ratios off and just control exposure with FEC with slave set to group A. Setting a single to B will confuse the metering because it expects there to be a group A and there isn't. Besides unless you enable A:B ratios, which makes no sense with one light, group B will not be in play. But if using two slaves with a non-firing Master you'd want to put one slave in A group and the other in B and turn on the A:B ratios in step two of the zoom button functions so you can control the relative power between them with the ratio indicator / control ring.
Moving a single flash off camera without fill is a recipe for harsh shadows, which get perceived as "hard" lighting. Leaving the Master on for fill will allow you to literally dial in the amount of softness you'd like in the lighting. Putting the master on a camera flip bracket positions it ideally for fill flash or single flash use. The downward angle of the light hides most shadows and puts those which are visible in places where they don't distract. So while a single flash on camera might suck if left in the hotshoe, raising it 12-18" over the lens makes a huge difference for single flash and perfectly complements an off camera flash.
Click the WWW button and look in the Canon section for my EX flash tutorials...
