When I mount a nfs share onto one of the folders in my home directory
(/home/naren/test) and open home directory in nautilus window, I can't
unmount the directory (/home/naren/test) which I had just mounted.

Steps to simulate:
a) Create a directory under your home, say "/home/naren/test".
b) From terminal, as super-user, mount a nfs share onto this directory.
   eg:  mount -t nfs <some-nfs-share> /home/naren/test
c) Now open nautilus window with home directory (/home/naren).
d) As root, try to unmount the mounted directory from command line.
   eg: umount /home/naren/test
Unmount fails with the follwing error "umount: /home/naren/test: device is
busy".

A similar case exists, where in, I was not able to unmount the mounted
cdrom or floppy.

Steps to Simulate:
a) Insert a data CD. It will get mounted automatically (usually mount point
is /mnt/cdrom). It gives a CD icon on the desktop and opens a nautilus
window displaying the CD contents.
b) Now open another nautilus window and browse to "/mnt" location. It shows
cdrom folder.
c) From the desktop, right click on cdrom icon and select Eject. 
cdrom will not eject and give error "device busy".

What I observed is, when I browse to a location in nautilus window, FAM
opens file descriptors for all the files/folders under that directory.
That's why "device busy" error is seen when I try to unmount, in the above
cases. For example,

$ umount /home/naren/test/
   umount: /home/naren/test: device is busy
$ lsof /home/naren/test/
  COMMAND   PID  USER   FD   TYPE DEVICE SIZE NODE NAME
  fam     31308 naren   22r   DIR   0,11  512    2 /home/naren/test/

I am not sure if this is an intended behaviour. Please evaluate it.