For those who wonders what this update is about; this is all I have gotten to know about this update, it seems the update is but small modification.
From a|w;
"Thank you for downloading mental ray(r) for maya 1.5. There is now an update that resolves a licensing issue with this software running on the Windows(r), IRIX(r), and Linux(r) operating systems. We invite you to download your free copy of this update, version 1.5.1 (...) "
I do not support Bush! Nor Saddam! Long live UN!