any ideas on how to do this? I work a lot with fondant, but this particular bride wants buttercream. Ideas would be appreciated.

I did something similar that's all in buttercream. I just crumb coated the cake and then used a large rose tip. I put the wider edge of the tip against the cake and spun the turntable as I piped and worked my way from bottom to top -- you can reverse direction (top down) for a effect more similar to the picture you posted. For a more wavy effect just move you hand up and down slightly as you pipe. Hope this helps!

http://cakecentral.com/gallery/1480346/wavy-lines

It can be done with buttercream, but make sure the bride knows that it won't look exactly like the picture. sometimes they don't "hear" that part when you're talking buttercream vs. fondant and they expect it to look just like the fondant.