Hi,
I noticed a difference in the file nu.mgz between FreeSurfer version v5.0.0 and v5.1.0 (running on Mac OSX10.6). It looks as if nu.mgz in v5.1.0 is an intensity scaled-down version of nu.mgz in v5.0.0. The release notes tell that a bug is fixed in mri_nu_correct.mni, I don't know if that may cause this difference.
Cheers, Ed