I would like to easily install and run (and maybe learn development with) the latest KDE off trunk (currently 4.4 beta/RC-ish) so which is the best/easiest distro for that?

I prefer to run Kubuntu (10.04) development version, next choice is Arch, followed by Fedora 12 (not rawhide) then openSUSE 11.2 (not Factory)...Mandriva being my last preference.

I can't seem to locate PPAs for Kubuntu nor the "Project Neon" nor the "KDE Four Live" CD images nor other repos for the latest 4.4 trunk code except for openSUSE's "KDE UNSTABLE" repositories (weekly snapshots).

So I am currently running openSUSE 11.2 with these repos and will try it out but do you guys have better suggestions (especially since I prefer Kubuntu) in the mean time?

GENTOO!
... I mean, yeah. OpenSUSE and Kubuntu are generally good choices, but your best bet is to get sources straight from upstream instead of relying on pre-packaged daily builds. You're best doing this under a new user - follow the guide here: http://techbase.kde.org/Getting_Started

I run Fedora 12 with the kde-fedora repo, unstable and testing enabled. It's never far behind. I currently have 4.3.85, which I believe is beta2.

There is also sidux which uses the Debian sid repository. The latest release (just a few days ago) included KDE 4.3.4

If you have a fast enough system then without a doubt Gentoo.

You don't have to manually download any sources either.
There is a KDE overlay with ebuilds for building from trunk and tagged releases, e.g. 4.3.85.

Very easy to manage.

yep, im already on 4.3.85 (4.4 beta 2) with Kubuntu 10.04 (Lucid) alpha... was hoping to get latest 4.4 trunk running... so i've now installed arch minimal, will svn checkout a little bit of KDE/trunk to see if i can get it up and running...

If you're going to compile anyway then just go with Gentoo.
You won't have to svn co manually and no problem removing, updating individual packages.

I'm running trunk for a while now and it's great and also very stable (for live sources) at the moment, since 4.4 release is getting pretty close.

OpenSuse with Factory repos (it works better than unestable repos) with New updates every day.