Sorry I've been absent from this forum for awhile and I know the thread is a little dated but I'll reply anyway
At the time I was using an SD1000, right now I've got an SD1100.
All Canon cameras that have a built in flash use this pre-flash and even my 40D did not offer the ability to disable this pre-flash. I have a 5DMKII and professional lighting, this is just for fun. I'd be nice if a guy could bring along a point and shoot and a few manual flashes (that have optical slaves) and get some nice shots with off camera lighting.
Right now the only way to trigger optical slaves with digital cameras that use this pre-flash is to drop $80 on a Wein "digital peanut" which essentially ignores the first pulse.
I tried using the "force manual flash" and setting the flash power but it still flashes twice (and yes I disabled redeye reduction
It might be that its just baked into the code that when the flash is called this pre-flash will always fire but if not it could open up some neat avenues for people on a budget.
If anyone knows any updates to my problem please reply.