I wrote a Perl script to rotate the RSS screen saver feeds every hour, using the plist hack hint as a basis. Currently, there are six feeds, and the script works best when the total number of RSS feeds divides 24 hours nicely. This insures that each feed gets an equal shot in the rotations each day.
An interesting note -- if the RSS screen saver plist file is modified while the screensaver is running, the display won't change to the next feed until the screensaver is reloaded (deactivated, then reactivated).
I set up a cron job to run the script every hour, and it keys off the number of RSS feeds and the hour value to determine which feed to display next. Forget the need to have a different screensaver for each URL!
[robg adds: I haven't tested this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050513131844902