Quote from Sheldon's site:

"More and more rear wheels now are built "half-radial" with semi-tangent spoking on the right side and radial spoking on the left. Radial front wheels offer mainly esthetic benefits, but half radial rear wheels are substantially more durable than conventional ones. The high amount of dishing called for to make room for more and more sprockets has caused an increase in spoke breakage on the left side of rear wheels. This is caused by metal fatigue.
A spoked wheel relies on having all of the spokes in constant tension. A highly dished rear wheel starts with very light tension on the left side spokes. The torque of hard pedaling can cause the left side "leading" spokes to occasionally go completely slack momentarily. Repeated cycles of tension and slackness cause these spokes to fatigue at the bends, and ultimately break.

With half-radial spoking, the amount of dish is slightly less to begin with if you run the radial spokes up along the inside of its flange. In addition, since there are no "leading" spokes, no amount of torque on the hub can reduce the tension on any of the spokes. In fact, if you have a customer who has been breaking left side spokes, "half rebuilding" the wheel into a half radial will solve the problem once and for all.

I used to think that this was exotic, cutting edge technology, until I happened to look at a couple of Model A Fords in a local parade. Their wheels were highly dished inward, and were laced in the same half-radial pattern, for the same reason."