raghu@booradley:~$ groups                
users lp floppy audio video cdrom
raghu@booradley:~$ ls -l /mnt/raghu/file
-rwxrwx--- 1 root users 8 2011-01-24 11:23 /mnt/raghu/file*
raghu@booradley:~$ cat /mnt/raghu/file
content
raghu@booradley:~$ echo "content-new" > /mnt/raghu/file
raghu@booradley:~$ cat /mnt/raghu/file
content-new
raghu@booradley:~$ groups
users lp floppy audio video cdrom
raghu@booradley:~$ su -c 'chown root:lp /mnt/raghu/file'
Password: 
raghu@booradley:~$ ls -l /mnt/raghu/file
-rwxrwx--- 1 root lp 12 2011-01-24 11:24 /mnt/raghu/file*
raghu@booradley:~$ cat /mnt/raghu/file
cat: /mnt/raghu/file: Operation not permitted
raghu@booradley:~$ echo "content-new" > /mnt/raghu/file
bash: /mnt/raghu/file: Operation not permitted

A similar bug was filed and fixed for NFS as bug 763777.

regards,
Raghavendra.