Hi mayaguy_dr,
yup it is "free"... as I recall it, it is an error and you would have to update Mental Ray which you can do at AW's site for "free"... 
Maya software updates
from AW's site -
mental ray for Maya 1.5.1
mental ray for Maya 1.5.1 resolves licensing issues with mental ray for Maya 1.5 on the Windows®, Linux®, and Irix® operating systems.
regards
Strarup
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Alex V. U. Strarup
One mistake can ruin it all...