Is there a way to set screensaver to always come on?

In System Preferences > Exposé & Spaces > Exposé

Set the Hot Corners something as you see in the screen shot:

This will allow you to immediately turn off the display or start the screen saver as soon as you want to leave your workstation without the concern of the display open for hours.

If I am reading you correctly you want to force the screen saver on when your machine is finished doing whatever it is doing and you are not there to know when it's finished. If so the previous answer won't help you. But related to that I stumbled on this neat little app. If you could find a way to execute it on some particular event it could probably satisfy what you want to do.

As an alternative suggestion how about forcing your machine to sleep as a function of your Energy Saver preferences? What you describe is that something is keeping your energy saver settings from turning on your screen saver. I've had a similar problem, but I wanted the machine to sleep, not screen save, and I could never find out what's keeping my machine from obeying the energy saver preferences (although I suspect it's my non-apple USB keyboard based on the kernel.log -- you might want to look at that by the way to see if you see anything related to your problem). At any rate I solved my problem by using a little app called PleaseSleep. It forces my machine to enable the sleep settings. Maybe this would work for you if you don't mind sleep instead of screen saving.

That seems unusual for it to be negated so quickly like that. You didn't nudge the mouse and dislodge the pointer from the selected corner? Once you activate the Hot Corner you have to leave the pointer in that position. Perhaps you inadvertently touched a key on the keyboard? Sometimes a slight bump of the computer table can undo it. Other than those couple of things I can't come up with an answer. Perhaps another member of the forum can help provide insight.

Meh replying to 2011 post. To anyone else that was having always awake issues. The way i fixed for customer was replacing keyboard. Past liquid damage caused a sometimes keyboard key press waking the computer up. It got so bad that the computer would not sleep even when closed. This can also be true for a damaged trackpad to cause phantom movements. With phantom trackpad there would be no evidence. Definitely would require some testing but you never know.