How can I display a web page as my Gnome screensaver?

try cutycap, a webkit based 'www-snapshoter':

CutyCapt is a small cross-platform command-line utility to capture WebKit's rendering of a web page into a variety of vector and bitmap formats, including SVG, PDF, PS, PNG, JPEG, TIFF, GIF, and BMP. See IECapt for a similar tool based on Internet Explorer.

then setup your screensaver application to pick up the snapshotted website pictures.

hmm ... maybe try the merger of what you both said ...

cron as "nobody" (or equivalent) every minute to run cutycapt into imagedir/date +nagios-%y-%m-%d_%T.jpg with auto-cleanup =

... * * * * * dir="/usr/share/admin/ScreenSaverDir" && a=date +$dir/nagios-%y-%m-%d_%T.png && CutyCapt --url=http://nagios.MyDomain.com/ --out=$a && && b=ls -1tr $dir | tail -1 && rm ls -1 $dir | egrep -v $b

so the most recent image is going to be the only thing in that directory but it will have a unique-to-the-second filename, guaranteeing that it will not be cached by the screensaver