A lot of iPhone dev types have known about the SBMobileScreenshotr preference for a very long time. What we didn't know was how to make it do anything. The software programming site RipDev solved the mystery.
After setting the preference
If you'd rather not edit your property lists directly, add http://repository.ripdev.com as an Installer repository and install Apple Screenshot Enabler. Warning: trying to remove the mod via Installer.app caused my phone to reboot. It just would not uninstall properly.