Photo: Dvorak319 on Flickr
Another option for using gripshifters on a drop bar is to use the Hubbub Adapter. This adapter mount in the end of your drop bar and is the right diameter to slide on a gripshifter of your choice. This works with Rohloff, Nexus, SRAM and Nuvinci shifters. A little pricey at $60, but I think this would be my prefered choice as I like having the shifter down at the end of the bar.
Of course if you've got some tools and DIY skills you can make a low cost adapter out of wood or metal and just glue it in the end of the bar.