The camera itself cannon trigger your flash in slave mode. The only Canon bodies that can do that, are the 7D and the 60D, via the built-in flash. The 1D cameras don't even have flashes.
As mentioned, you can use a 580EX or the ST-E2 (speedlite transmitter) as a master, to trigger your slave. Or you can use the TT pocket wizards.
Another option is to just use a basic radio trigger. Anything from the cheap Cactus ones to the expensive Pocket Wizards ones. You won't have TTL metering, but you will have simple, effective flash triggering.
I keep a 580 on camera as master; then slave a 580 and up to 2 430's. Don't forget that you can change the ratios master 1 : slave 2 / master 4 : slave 1 / etcetera