The development themes are all done in Inkscape and a couple of themes that are based on it should hit the Ovi store soon (well, when Ovi store goes live). I hope we can get at least one template open sourced, and I am quite optimistic about that (work is under way).

Inkscape is actually really good for making sharp looking widgets, if you know exactly what you want to accomplish. Photoshop is fine too, and the filters can be a very powerful helper. Unfortunately Gimp doesn't have anything comparable to these filters, so I personally wouldn't enjoy working on a theme template with Gimp.