This summer I had the same problem that NZcyclist and skdsl are describing. My spokes were breaking at the hub flange nearly every ride even when I wasn't in a rough section of the trail. My wheel wouldn't go too out of true each time, so I could ride it back to the trailhead carefully. What ended up being the problem was that the factory wheels that came on the bike were of a low quality and weren't built correctly to begin with. The spokes were loose enough at the flange to allow a small amount of movement to occur at that spot. The spokes were also no-name to begin with. Eventually, the spokes started to snap along the hub from the repetive stresses on those spokes from riding the wheel with the problem, and I realized what the problem was. I bought a new handbuilt wheel from my LBS with quality components and haven't had a problem since. The problem only occurred on the non-drive side spokes of the rear wheel. It's at least worth checking the wheel to make sure it is spoked and tensioned properly.
-Moab