Freesurfer (http://freesurfer.net/) also provides many matlab functions in $FREESURFER_HOME/matlab, including "load_nifti".  I particularly like this function because it works with .nii and .nii.gz.  For example:

foo = load_nifti('some_file.nii.gz');  

will return a structure 'foo', with many fields corresponding to header information, and your image data can be found in foo.vol.

Hope this helps,

Andrew