This has been bugging me for a while and I've ignored my ignorance for long enough now.

For OS2008, what is the repo settings that I should use to get the full Extras-Devel.

Catalog Name: (Does the name matter at all?) I use "Maemo Extras-Devel"

Web Address: http://repository.maemo.org/extras-devel/
I guess the only setting I'm ok with

Distributions: I've used "Diablo", I've used [blank]...what should I have here?

Components: I've used "free non-free", I'ved used "free", I've used [blank]

I basically just want to make sure I pick up all diablo-related extras-devel items.

http://repository.maemo.org/extras-devel/dists/diablo/

So the name doesn't matter, dist should be diablo and free non-free should work

Interesting, I've used the 'wrong' URL all this time?
I guess having 'diablo' in the Distribution field still redirected it to the /dists/diablo/ folder - and assuming that that's the case would that mean that with that URL the value 'Diablo' is not really needed in the Distribution field? (I'm mostly asking for undestanding's sake rather than being picky)

Debian repos are not my forte, but yes, the correct URL should be just "http://repository.maemo.org/extras-devel". The name is strictly for your reference, and has no bearing on anything in particular. The "dist" field should be diablo in this case - it stands for "distribution", IIRC, and is usually set a name/number/code for the current distribution version you are using, so that multiple versions can be in the same repository system at a time. (Just like Ubuntu still provides packages for older releases in the same place as the latest release.)
The "components" field is a way that the repo maintainers have of logically separating packages in a repo. In this case, the "free" section contains* "Free", e.g. Open Source packages, and the "non-free" section contains Closed Source packages. If you don't want proprietary software, then you can just omit the "non-free" bit. (You can do the same for Extras as well, if you care to - it works the same way.) Some repositories will have different sections names, such as (off the top of my head) "user" or "main", so it won't always be the same.

*I think this was the intent, but I can't actually vouch for that. You'd probably want to ask someone like Jeremiah or Stskeeps if you have specific questions about how [the] repos are run.

If the distribution line is left blank, it defaults to the current distribution installed on the device. Obviously, Diablo in your case. You only need specify the distro if you'd like to live dangerously and install applications from a non-current distro, say Bora or Chinook, but explicitly specifying Diablo doesn't hurt anything.

Thanks all.