Bruce Stephens
I'm running Debian unstable with gdm, and I just have an executable script ~/.xsession which (after running ssh-agent, etc.) runs xmonad. As, before, it ran ion3.
Shows how long ago I first started doing this: the critical step is to choose the Xsession session when logging in to gdm. I imagine kdm does something similar. By default they'll give you a typical gnome or kde setup (modified by local files), but you can ask for the traditional option which mostly involves running ~/.xsession, if it exists, and some default script otherwise. There are (typically) some stumbling blocks to do with PATH and so on, but I imagine most people could resolve those easily enough once they'd at least got the right window manager running. [...]