(Update - 17.12.06: This guide works in Edgy as well.)
As you're probably aware, the gnome-screensaver GUI that ships with Dapper's GNOME 2.14.1 doesn't give the user any control on individual screensaver settings. I had previously posted a guide on replacing gnome-screensaver with xscreensaver, but it has a few drawbacks (which I listed in the linked thread) that some users may find deterrent. Hence the reason for this guide, which does the job in a more elegant but maybe slightly less convenient way.
This will show you how to set preferences for your screensaver by figuring out the required command line with xscreensaver and putting it manually in the screensaver's gnome-screensaver theme file. It takes a bit more manual work, but I prefer this method over the other one since it doesn't have the drawbacks of replacing gnome-screensaver altogether, and it's more "technically correct".
To compare, the other method is a bit more convenient if you adjust settings very often, but this one doesn't break anything and doesn't touch the Ubuntu defaults. It's up to you to decide which one to use.
1. Install xscreensaver.
Note that unlike the other method, you won't actually be running xscreensaver full time; you'll only need its preferences interface to adjust screensaver settings to your liking and preview them.
sudo apt-get install xscreensaver
2. When you want to adjust the settings of a particular screensaver, launch the xscreensaver preferences dialog with and when asked whether you want to launch the daemon, click "Cancel". This is important, since running both the xscreensaver and gnome-screensaver daemons together is very likely to cause unwanted behavior.
Choose the screensaver you want, click "Settings" to adjust its settings, and click "OK" to go back. In the Advanced" tab of the main window adjust image and text manipulation settings as you like. Check the preview area to see if it works the way you want, or hit "Preview" for a full screen preview. Once it's working as you want, click "Settings" and then "Advanced", and copy the string in the "Command Line" string box.
3. Open the gnome-screensaver theme file that corresponds to your selected screensaver in /usr/share/gnome-screensaver/themes (in edgy this is /usr/share/applications/screensavers). For the Phosphor screensaver you'd have to do
Locate the line that begins with "Exec=" and replace the command that follows it with the command you copied from xscreensaver's "Command Line" box.
gksudo gedit /usr/share/gnome-screensaver/themes/Phosphor
4. Open the gnome-screensaver preferences window (System / Preferences / Screensaver) and choose your preferred screensaver there.
5. Hit Alt+F2 and enter
to test if the screensaver is behaving as you like.
6. Optional: Install the extra screensavers in the repositories that don't come installed as default.
For convenience you may want to use Alacarte or tweak the files in /usr/share/applications to create a separate Applications menu entry for xscreensaver called "Screensaver Settings" and rename the existing "Screensaver" item to "Screensaver Selection".
sudo apt-get install xscreensaver-data-extra xscreensaver-gl-extra