Another way to look at it is the difference between the last time and the first time divided by the number of times it's in the file minus 1.
This should get you the same results minus the headache of the first post.
Code:
SELECT MachineName, (DATEDIFF(minute, MIN([Date Time]), MAX([Date Time]))) / (COUNT(MachineName) - 1) As MTBF
FROM SomeLogTable
GROUP BY MachineName
ORDER BY MachineName