I've done a couple of gradient mesh works using Illustrator. The G-mesh tool in Illustrator works to create organic looking work, but a drawing can become extremely tedious.
Some guys have created what appears to be the perfect solution with Diffusion Curves. It's difficult to explain how it works, so watch the video:
The tool itself includes a few other clever ideas that I wouldn't mind seeing in Inkscape, such as the "active contour" feature that attracts curves to the contour of underlying image.
I haven't yet tried the tool, but you can download it at the Diffusion Curves publication page.