Steve,
The MAT is a tool that has been used in various formats for several years.
I have even modified it further since I made this post as I believe that business tools should grow with your understanding, or you will simply outgrow them.
Now I use MAT as Milestones/Assumptions/Tasks. The milestones are the key things that must be accomplished to achieve the major goals. I now include dates that I plan to have these milestones completed.
The assumptions are the major assumptions or perceptions that I have made upon which I am basing some of my decisions. Some assumptions may change or they may be based on temporary conditions. I put a date on these also. This date is the date that the assumptions are effective or may change.
Last, but not least is a list of the tasks that are needed to make the milestones happen. I put dates on these also. These dates help to hold myself or the team accountable.
For more info on this subject check out the ebook 4Steps2 Transition Your Business and its accompanying workbook located at
http://www.4steps2.com.