It occurs to me I may have described the problem poorly--here's a screenshot of the problem with the Linux version (On windows the non-wrapped one does work, but the wrapped one looks identical.)

Without using setWrapWidth: https://dl.dropboxusercontent.com/u/187059/ComboBox-nowrapwidth.png
With setWrapWidth: https://dl.dropboxusercontent.com/u/187059/ComboBox-wrapwidth.png

I'm using DMD 2.064 on both OSes, the latest Git update of GtkD1, and the latest official GTK 2.24 runtime on windows, the latest Ubuntu 13.10 GTK 2.24 package. 32-bit on Windows, and 64-bit on Linux. And this problem doesn't show up if I use a gtk.ListStore as the model for the ComboBox; both wrapped and unwrapped popups render fine in that case.