The most excellent normal map plugin has been updated to work nicely with Gimp 2.7 / 2.8 , previously (as it was only for 2.6.x) the 3D preview was crashing Gimp 2.7/2.8 :

http://code.google.com/p/gimp-normalmap/

That's a very cool plugin. I haven't used it before now so I will need to have some fun time to test. One issue on my machine, (Win32, Gimp2.8). The preview window seems to update fine, but if I hit the 3D preview button it generates a Windows and Gimp error message and crashes.

A quick go at it.