I was not able to access the VirtualBox shared folder, so I made the user part of the vboxsf group. But, still not able to access it. Rebooting the guest resolved the problem. Is it a bug? A reboot is not required to add a user to a group.
sudo usermod -a -G vboxsf praveensripati
There was similar query here
BTW, I am using 11.10 and don't remember seeing similar behavior in 11.04.