What is /Library/Preferences/com.apple.loginitems.plist, how to edit it?

The "com.apple.loginitems.plist" file may be new - I don't recall seeing it prior to Leopard. I haven't looked into it in detail, but it (together with the familiar "loginwindow.plist" file) appear to be configuration files for "login items", one difference being that with the "com.apple.loginitems.plist" file, the launch order follows the order of the items in the list, unlike the "loginwindow.plist" file.

When these files occur in a user's "~/Library/Preferences" folder, they apply only to that user (eg. "login items" added using the "Accounts" pref pane write to files in that location). In contrast, items in the boot drive's main "/Library/Preferences" folder will apply to all accounts.

The "/Library/Preferences/com.apple.loginitems.plist" file isn't something that exists in the default configuration. If you don't need the items listed in the file, it should be possible to remove it without any issues. Or perhaps just remove it from "/Library/Preferences" and keep it somewhere else in case you find things don't work as expected and you need to put it back...

You need PropertyListEditor to edit these files. The easiest way to get a usable copy is to down load the latest Server Admin Tools update from the Apple Software updates page. Mount the disk image and open the Utilities folder on the disk image. Copy the application into your /Applications/Utilities folder.

Also included with Xcode Tools which is an optional install on your install disk.