I have two hard drives in my computer divided into four partitions
hda1 is swap and has a size of about 2Gb
hda2 is ext3 with Slackware 12.0 about 15.0 Gb (a test partition)
hda3 is ext3 with Slackware 12.0 about 120Gb (What I normally run)
hdb1 is ext3 and is used to backup sources, packages and other media
Now here's the question. What permissions should I give the media I backup. If I switch to another distro my UID's and GID's will most likely be different. I want whatever I add to be pretty much permanent with permissions of either 444 or 544 for scripts. Is there a way I can do this and still let everyone have access to the files?

That depends on what files you are backing up and why. If you are simply concerned about user files, then just backup /home and don't worry about the UID, GID problem. Unless you have hundreds of users it isn't difficult to fix (or avoid by assigning old users the same uid/gid).

Generally if you are backing up the OS, you would simply not worry about this issue either, because the purpose of an OS backup is to restore the machine as it was at the time of the backup. Practically speaking, other than user files, you will not be able to use much from a backup if you are transitioning from one distro to another. At least not directly, because different distros move around where various settings and files are stored in enough cases that it would be more work to fix than simply rebuilding from scratch.