If you have moved your Applications to your mobile directory (/private/var/mobile/) you might have problems with your installer.app permissions.
Howto move your applications from your small root partition :
- cp -r /Applications /private/var/mobile
- mv /Applications /Applications.old
- ln -s /private/var/mobile/Applications /Applications
CD into your now-moved Applications folder and do the following :
- chmod +s Installer.app/Installer
- chown -R root:wheel Installer.app
This worked for me. YMMV, of course.