I'll not enter this fight again, except to say that what humminbird posted isn't wings frozen, nor is it in flight, nor is it sharp. I'm still flabbergasted he continues to post this as an example of a good photograph.
As for the technicals of what is being asked, as well as the lens:
1) you don't *want* ambient light...you want to underexpose 2 or 3 stops and set your shutter speed to max flash synch (1/250th as mentioned above for 20D). You are not stopping the wings with the shutter speed at all..it is *entirely* flash. I'll explain in point #2 below. Ambient light in this instance just ends up ghosting the wings as you end up getting two exposures. If there is enough ambient light you will get an exposure for the ambient, and another for the flash. This can be a cool effect, but generally it isn't wanted.
2) By setting the flash on manual and setting it to 1/16th or 1/32nd power, and then underexposing the ambient, you are using flash as main light, and thus the speed of the flash is what is used as your shutter speed. The reason for 4 or 5 flashes is that at 1/16th power, the flash only throws light maybe 18-24 inches, so you need multiple to light up the entire bird evenly.
3) Given the small distance the flash will go, you need to set up flashes around the bird...and if you plan to trigger remotely from an on camera flash, you'd be best served to have a better beamer to extend the distance it will throw light. For my hummingbird photography in the past (since I don't have 4 or 5 flashes yet), i've used a Canon 550EX on camera, and rented 3 or 4 other 550EX's from a local camera store. You can generally get them for 10-15$/day. This way I don't need any sort of synchronizer, as 550EX's (and the new 580EX's) can all be remotely triggered, so by setting the flash on camera as master, and the flashes off-camera to slave, and making sure they are all on the same channel, you will trigger all of the flashes when the first one goes off. The need for the better beamer in this case is to make sure the light gets to the flashes. I found this wasnt a problem using a 550EX + BB at 6-8 feet or so at 1/32nd power even.
4) As for the lens...hummingbirds *are* fearless, and generally pay people no heed no matter where they stand. I've had them land on feeders and feed while I was re-filling a feeder. I take almost all of my hummingbird shots with a Canon 70-200mm f/2.8L USM. 200mm is generally enough for most anything, and if I need more, I can always use my 400mm f/5.6L. Point being you don't need a big lens. You are also going to be stopping down to f/16 or below (i've taken many shots at as much as f/25) to render the entire bird sharp. At such close distances, DOF is small, so you need the aperture as small as possible...f/13 generally is the minimum, though f/16, f/18, f/20, f/22, and f/25 are the f/stops I use for hummer photography.
I hope this helps.