it's great that someone has actually posted the equations for torsion, but I want to weigh-in and agree with the churros being the 'worst shape' for transmitting torsion.
In an engineering class somewhere along the line, I learned the 'membrane analogy'
http://en.wikipedia.org/wiki/Membrane_analogy for torsion in a shaft.
Essentially, the calculus is the same for an imagined 'membrane' (think square of latex of a balloon) that's stretched over the end of the shaft. So if you could stretch a balloon over the end of the shaft and then air it up, if it holds alot of air then the shaft will withstand alot of torque. But if the shaft has cuts in it--like a churro--then it won't hold much air and hence not much torque. This is also true for shafts that have been through-drilled with a pin--if you do that, you cut the torsional limit to about half to a third of what it would've been. Also shows how keyways and/or splines are good because they transmit torque without hurting the torsional limit of the shaft very much.
Also, in the membrane analogy, if you air up the balloon-stretched over the end, the slope of the transition indicates stress. If it produces gentle-slow changes in slope, these indicate the stress will be even. If you stretched the balloon over a churro, the slope in the vee-grooves would be very steep, indicating high stress. This also shows how if you use a square-section for torsion, the failure will be in the middle of the sides, not at the corners.
Will it fail? I don't know--it would be difficult to estimate the loading. Is it a bad shape? yes.