Lucy,
I use my Canon 580 EX as a fill light outdoors a lot -- especially when shooting show formals outdoors. It softens the shadows and makes it easier to bring them up later in PhotoShop. I set up the flash to balance with the available light rather than overpower it. Canons have a Custom Function that allows you to choose which way you want to use your flash unit.

I also use my AB400s with a Vagabond II outdoors when I want to control the light completely. I meter the highlights and shadow areas and then set up my lights to fill in the shadows while I set up the camera to underexpose the highlights slightly. Camera settings are manual. That's pretty tricky stuff and it takes some trial and error (mostly error ) but with digital, the viewing time is immediate so you can tell what's working when you make your changes.

For me, the trick to using flash outdoors is not to make the images look as if you have. Lots of folks use flash to properly expose the subject(s) but then underexpose the rest of the frame so much that it's obvious that they've used flash. Personally, I prefer to balance the lighting so it looks as natural as possible.

Jim