Using ubuntu 10.04, there is a file
/etc/acpi/sleep.sh. That file contains, among others, the command
pm-suspend, which is the suspend command. I assume that when you change that line, you can invoke the screensaver instead of suspending. Note that the file does some checks and bails out on some conditions which I do not grasp completely.
On your system the file might be named differently.
I did following search:
grep pm-suspend -r /etc/
Alternatively: you could replace the command
pm-suspend with a script starting the screensaver. I really really do not recommend this. The package manager will not like this, and any other situations where pm-suspend is expected to work the way it does will break.