Apparently it's because a previous installation (within Installer) hasn't completed. The solution to my problem was to connect to my Touch and delete the applications that hadn't installed properly the last time, delete Installer.app, then replace with the previous version. There's a link to the previous version in this thread:
Well, here's another thing to try that worked for me. A little digging and I determined that the update ran my root fs out of space. I did the trick of moving my fonts and creating a sym link.