What?! Using flash in landscape photography? That's blasphemes! I know some people only shoot in natural light and the introduction of flash in landscape photography is against all rules, but if it improves your image than I have no problem with it, as long as you can't tell you're using flash. I just returned from a trip to Mt. Rainier. The early morning light strikes Mt. Rainier long before the light strikes the colorful wildflowers. Now I could use a ND filter and do a long exposure, but the flowers would all be blurry and lack the "color pop" you witness in person. In comes some fill flash. Use manual exposure and meter the blue sky. Then turn on your flash. I start with minus 3 stops of flash exposure on TTL and see how it looks. If I need more flash I will raise it by a half a stop. I do this until the foreground looks good. I also have a Stofen Omni-Bounce on my 580EX II which helps scatter the light. I have my flash connected with a cord, I try different flash angles to get an even light on the flowers.
In review the flash does a couple things for my image. It provides a minimal amount of light on the flowers to help even out the exposure, it "freezes" the flowers to keep them sharp during a longer exposure, and it also makes the color of the flowers "pop" whereas they would be dull without the flash.
Experiment with your flash next time you're faced with a difficult situation and see what you can come up with, but remember the key is to make it appear that no flash was used. If you didn't tell anyone they would never know.