Shaun
Two real options:
1) Use a long off shoe cable to get the flash off camera - canon and 3rd party companies do them - Canon OC-E3
Canon EOS Dedicated TTL Off-Camera Shoe Cord OC-E3 Review
2) Buy the STE-2 wireless controller from Canon, and use this to wirelessly trigger the 550 .
The STE-2 is actually IR and not wireless, so works well indoors but fails outside in sunlight!
Canon Speedlite Transmitter ST-E2 Review
If you're shooting outside - you'll want to look at proper wireless triggers - something by Pocketwizard would be ideal.
You buy one for the camera, one for the flash - and you get full ETTL functionality wirelessly
The latest unit looks very promising - Pocket Wizard 801-130 Plus III Transceiver
There are cheap knock-off wireless triggers on EBAY - not sure how much of the ETTL functionality you'd get though.
Or - if you just want to fire it randomly while the shutter is open - there's a manual 'fire now' button on the flash unit itself.
Either the Pilot or the button to the right of it here - can't remember which, but it works as a full power flash each time you press it.