Each IPSW file is really a zip archive containing several files. The largest of these is a disk image containing all of the files in iOS. This "ramdisk" file is a what is copied to the phone's RAM when you installing the OS. This file is usually encrypted so that it is not possible simply to open it on your computer and have access to the entire iOS file structure.
In this latest version of the firmware, the ramdisk image was (probably inadvertently) left unencrypted. This means that anyone can look at any file in the OS simply by opening the file on their Mac or PC.
This doesn't really affect the jailbreak process, although it may make some software development a little easier.
