You don't really need any special ruler. I made my first one using rectangular rulers that had 60 degree markings, and the OBW book explains how to make them this way. It takes a little more care, but the rulers are pretty pricy, so you may want to try the technique before investing the $$ in the ruler.
However... for my next OBW, I have purchased a 60 degree ruler, because that will make it easier - and I plan on making several more of these quilts, when I find the right fabrics.
Just a note - Marti Michell just came out with a 60 degree ruler, and hers measures along the side of the triangle rather than the height of the triangle. Either type of 60 degree ruler could be used for the OBW, but not necessarily for other patterns, so be aware of the difference before you select a ruler. This is her site, which explains the difference better than I can:
http://frommarti.com/60deg.shtml (The OBW book gives directions for using rulers that measure the height of the triangle rather than its side.)