I went digging further and found the answer, it's not a KDM problem but a lesser-known caveat of the nouveau driver itself. Nouveau's built-in 3D support is limited to xorg 1.5 and higher, to get it going on 1.4 you need to also install the gallium driver (which is still very unstable). That provides the missing nouveau_dri.so file. 

I'm not sure why from kdm XFCE also tried to load it but once I added gallium (as per the nouveau wiki I didn't install it in the system but rather added ld.so.conf entries for it and just symlinked the dri_file into the xorg tree) things loaded right.

At that stage KDE and XFCE could load, but there is one glitch which other nouveau users should know about: at least for now no GL functions work on the 2D level with it. It causes a completely lockup (so completely I suspect there's a kernel panic behind the scenes) if you are lucky, and a xorg crash if you're not. 

This means you need to hand-remove any compositing entries from your kwinrc file if they are there before KDE will load, and you cannot open the screensaver or desktop entries in control center (the compositing test in the latter and the GL screensavers in the former cause crashes).

That said, without compositing and avoiding those entries, KDE under nouveau with remarkably faster and more stable than under proprietory nvidia driver. Nouveau has a ways to go before it will be a real replacement (tv-out is barely even started)  but it's coming along and it may already provide a better KDE experience than the nvidia driver for many users (at least, if you don't miss 3D acceleration for stuff like games). Twinview and nvidia-style xinerama doesn't work, but the xrandr 1.2 support solved that easily enough and I could get full dual-head support with just a few changes to my xorg.conf file.

I had previously used the fbdev driver with the vesafb kernel driver with some success, but it doesn't support dual-screen even with plain xinerama and it's set of available resolutions is rather limited. Though it lacks any compositing support it was by far the fastest KDE4 I have yet had. I would still like to test the rivafb kernel driver with fbdev and see how that compares as well.

Ciao

A.J.