Yes. Simply apply a transmap. And make sure Transparency Display is set to Actual, rather than the default of 90%, in your preview render settings.

If it still isn't working, there might be a compatibility issue with your graphics card or drivers. Try the SreeD option instead of OpenGL.

Addendum:

The cartoon display modes still won't use transparency, and that new Toon Outline option for Firefly will persistantly draw outlines around transparent materials, whether you want them there or not.