Rajeev Nair: You said: "Any user who knows the admin password can install updates"...
Did you actually test this with a user who is not in the admin group? What "admin password" do you have in mind, since Ubuntu systems do not normally have a root password? I think you are mistaken. Only users in group admin can install package updates, as far as I know, and as far as my own testing here on Ubuntu 9.04 Jaunty and Ubuntu Karmic Alpha4 shows.
Rajeev Nair: You said: "Any user who knows the admin password can install updates"...
Did you actually test this with a user who is not in the admin group? What "admin password" do you have in mind, since Ubuntu systems do not normally have a root password? I think you are mistaken. Only users in group admin can install package updates, as far as I know, and as far as my own testing here on Ubuntu 9.04 Jaunty and Ubuntu Karmic Alpha4 shows.