There's a few things you can try. First; Go in via SSH and recursively set the permissions of the Installer.app directory to 755.
If this doesn't work, go into the /private/var/mobile/Library/Installer/ directory and rename or remove the following files:
LocalPackages.plist
RemotePackages.plist
Restart your iPhone. Removing these files will force installer to refresh sources, and pull down new package information.
If neither of these work, I'm afraid you may have to restore. Perhaps someone else has a better idea... Use iLiberty;
Ziphone is dangerous.
Cheers,