While there are great improvements in the brush engine in Gimp 2.4, such as scaling and jitter, it still lack the native ability to rotate a brush while you paint. You can, however, create an animated brush (.GIH) that track the angle of your motion and uses a different brush image depending on the angle. This can be done by duplicating a brush, along with a bunch of rotated copies of itself, and saving it out as a .GIH animated brush. Unfortunately, this is a pain in the butt to manually set up an rotate. I was having troubles with GAP (which I was told would do what I wanted). I also couldn't get python-fu working on my windows machine so this plugin[?] wouldn't work for me.
There is a way to script for Gimp using tiny-fu, which is a small scheme interpreter, but I had never used scheme before. (Very RPN'esque, by the way.) After banging my head on it for a while, figuring out how the Gimp procedure s can be accessed, and generally copying allot from other plugins, I finished my plugin. The download link is at the bottom of the page.