Here is a great account of a winter JMT trip on skis. These people were expert winter ski mountaineers and it was still ridiculously dangerous. FYI, if you don't know, Doug Robinson and Galen Rowell, who are mentioned in the first paragraph were legendary climbers in Yosemite. Rowell was a renowned photographer who was killed in a plane crash a few years ago.

http://www.jamesalutz.com/index_JMT1.htm

http://en.wikipedia.org/wiki/Galen_Rowell