mount point labels

I'm attempting to create the labeled mount point with the following command:

mount -t nfs -o context=system_u:object_r:import_file_t:s0 nas:/vol/home /home/SimulatedImport/output/home

The mount point is created without any errors, but the label that I specify in the mount command is not used. Instead of system_u:object_r:import_file_t, the context of the /home/SimulatedImport/output/home is system_u:object_r:nfs_t:s0.

ls -dZ /home/SimulatedImport/output/home

drwxr-xr-x root root system_u:object_r:nfs_t:s0 /home/m252/SimulatedImport/output/home

I'm running RHEL5 with a policy built as mls off of the targeted policy.

Does anyone know why the context label is not taking?

Thanks