http://www.youtube.com/watch?v=UGSnLGgeai8
Here's some footage of Collin McRae. In the first example of understeer, he induces it. Although judging by that, it's probably not the best technique for rallying
Some great driving though, from the legend himself!
I think it can be used effectively, though.. But I think it'd be more of a 'worst case scenario'. In fact, I'd think it'd be faster (for me) to just correct the oversteer on turn in (just enough to stop the rears from totally losing traction) and have a bit of a slip angle/drift through the turn, rather than inducing understeer then waiting for it to turn.
The only time I ever induce understeer is, as Dave said, when the front tyres are getting too cold in a stint. But that's not to correct oversteer. When I do it, i just brake too late, and turn in hard, about 180 degrees of ratation on the wheel (into a hairpin or other tight corner) until you hit the apex. It's sometimes as fast as taking the turn normally, but never as fast as taking the turn with the rear stepped out a little on turn in I don't think.
Actually come to think of it, I think Rudy uses this technique a lot, not sure if he knows he does it or not
But I think he does it to wash off speed into a corner, rather than taiming the rear end.