Seems we changed the default kde wallpaper, but forgot to ensure that it was actually present (and usable). doh.
kdebase-workspace-4.6.0-8.fc15 has been submitted as an update for Fedora 15. https://admin.fedoraproject.org/updates/kdebase-workspace-4.6.0-8.fc15
nice-n-easy fix, hopefully to get included in f15alpha...
kdebase-workspace-4.6.0-8.fc15 has been pushed to the Fedora 15 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with su -c 'yum --enablerepo=updates-testing update kdebase-workspace'. You can provide feedback for this update here: https://admin.fedoraproject.org/updates/kdebase-workspace-4.6.0-8.fc15
kdebase-workspace-4.6.0-8.fc15 has been pushed to the Fedora 15 stable repository. If problems still persist, please make note of it in this bug report.