Hello,
I arrived in Canada on a two year ago on a work permit and am still employed with the same employer. I have now renewed my work permit with the same employer for another two years to Feb 2013; as well I have submitted to Detroit office to change my single entry visa to multiple entry visa. I would like to go and visit my family for a couple of weeks. I have confirmed with my employer the time off. What would I need in order to re-enter Canada? Is the multiple entry visa sufficent? Or do I need to carry more documents with me? Thank you.

At a minimum, you need the valid work permit, valid entry visa, and valid work permit. Additional evidence of your ongoing employment with the employer in the appropriate occupation could be beneficial but would not likely be needed.