The Michigan DNR reports that low-lying areas on trails in parts of Michigan are not in great shape, due to the warm winter weather and minimal snowfall of late. They report that most trails in the UP are in pretty good shape and remain open, but caution riders to be vigilant about changing conditions.
The trails in the northern Lower Peninsula are mostly still open, but they're in 'fair to poor' condition. Down here in southern Michigan, forget about it - trails are closed.
Check websites of local snowmobile grant sponsors or the Michigan Snowmobile Association for trail updates.
Information is available at http://www.michigan.gov/dnrtrails