Check the X11-extras README on "if you are building from darcs"
I went back and called autoreconf, then wiped out the contents of the dist/ dirs inside the repos (both for xmonad and X11-extras). The I configure, built, and installed X11-extras and xmonad. Same behaviour again, so I tried wiping out the install in /usr/local/lib/X11-extras and did it all again, still same "single workspace across two screens" thing going on ...
Just double check you're not linking against the wrong things.The process would be:
darcs get --partial X11-extras autoreconf configure (check it finds Xinerama) build that install
get an fresh xmonad from darcs configure ; build ; install
See the mailing list recently for another case of Xinerama being lost due to linking against old versions of libs,
http://thread.gmane.org/gmane.comp.lang.haskell.xmonad/379
(Note that nothing's changed that would break Xinerama handling, so its alsmot certainly due to linking against the wrong version of the lib. runhaskell Setup.lhs clean is advised for any change in dependent libraries, like X11-extras).
Cheers Don, I grabbed both all 3 repos again from scratch and it seems to be working fine now :-)