Kristen, not to sound harsh or anything, but I'd shoot with the light, not against it, regardless of the mountains [img]/images/graemlins/smile.gif[/img] I know that's impractical, but I just had to say it: I generally hate the "flash against the light" look so popular today.
Having said that, I find--when I have to--that ETTL2 and Canon's latest flashes (580) spot on when I throw a couple of them into a portable umbrella as an "outside" single light source (with the Canon remote trigger--I drop the background exposure as a kind of vignette anyway). The light is nice and soft, and you can still see the background [img]/images/graemlins/smile.gif[/img] Something to try, anyway, but you'd need (at least) another flash and the STE2.
Anyway, the large umbrella I was talking about is an 86" (!) Westcott optical white; best location modifier I ever bought, and keeps people dry too, if need be [img]/images/graemlins/wink.gif[/img] Certainly big enough for most wedding parties; you just need someone to hold it out of shot.
http://www.fjwestcott.com/products/u.../oversized.htm