Using Ubuntu 12.04.
I'd like to know if it is possible to set a different password for Ubuntu screensaver or not?
It is possible. There are two ways of doing it:
Option 1: xlock - this program can lock your screen and has an option where you can specify the unlock password.
create encrpyted password:
lock with custom password:
The ubuntu package for xlock that I tried didn't have the -cpasswd option, so you might have to compile it from source to use that option.
Option 2: xscreensaver is an alternative to gnome-screensaver. There is this howto that show you how to set it up in Ubuntu. Once setup you can create a dummy user on your machine and set whatever password you want for the screensaver through that user. Then you run the screen saver as that user, that way when the screensaver comes up it will ask you to enter that users password. I tried and tested this method just now it works.
allow dummyuser to access your screen
run screensaver as dummyuser
You cannot have a different password for the screensaver unlock, from your log-in password, no.