As expected, the iPhone 1.1.1 Update will remove all 3rd party applications when you update.
Simply reinstalling Installer.app will not work as the Installer.app application is firmware specific. At present only supports firmwares 1.0, 1.0.1 and 1.0.2.
We are hopeful that nullriver will be able to update their installer to support 1.1.1, but there are no guarantees. If 3rd party support is important to you, you should wait to update your iPhone to 1.1.1.