From the gimp-devel mailing list:

For those of you who don'tṫ follow trunk closely or the irc channel, I can't resist the urge to tell that Alexia Death and Mich have been working on brush dynamics over the last few days - and have thus far added brush rotation. It now works both as a fixed factor and following painting direction.
(as any brand new feature, it still buggy and error prone, but they are actively fixing everything)

-Rob A>