In 2007 I met a whole lot of southbounders in February who were finishing their wintery hikes. But more interestingly, I followed the register entries of a northbounder who had started in October and hit katahdin in early June. It wasn't all in winter, but it was pretty mind blowing anyway. If you ever find some 2007 registers, look for Bookworm. His stories were pretty great– some days he'd make it less than two miles in the snow, he'd go weeks without seeing a single hiker, and he'd surprise the heck out of trail town people because they never expect anyone to come to the hostels and such at that time of year.
Didn't someone try the Long Trail last winter? I never heard how that turned out.