I've recently found a possible fix for this issue
By default, the Linux implementation of NFS uses a privileged port (i.e. a port lower than 1024) for NFS and denies - for security reasons - cooperation with NFS clients using a non-privileged port. This is exactly what Mac OS X NFS does. So you have to tell the OS X NFS client to use a privileged port when trying to mount an NFS share from a Linux server. This is done with the "-P" option:
mount -o "-P" 192.168.1.75:/usr/export /mnt/share
Does the "nfs://" method use a non privilaged port, where the mount command is now using a privilaged port, by default?