There have been only two or three occasions in my career when I've needed to create a curvy vector line for a design in Photoshop. Each time it took me much longer than I anticipated. I'm not sure why, but what seemed like obvious approaches before always turned out to simply not work as hour upon hour went by.
I finally found a way! The best method I've found so far is to use the Line Segment Tool in Illustrator to create a straight line. Then use Filter > Distort > Zig Zag. Be sure to select "smooth" for the line. Be careful not to use the ZigZag inside of the Effects dropdown but instead confirm you are in the Filter dropdown before applying the distortion.
You should be able to copy/paste into Photoshop as a smart object now. I'm glad this has turned out to help so many other folks!
I am very pleased with the outcome of this project. A solution that compensates for critical deficiencies in IE support for CSS was provided by John, enabling us to push ahead with a strategy that may not have been possible otherwise. My advice to you if you're considering working with John Barrick on a project is: Do it! I intend to continue working with John on this and other future projects.