I recently installed BSDI on a system with this controller.
  BSDI supports the 2940 just fine, but I had to get updated bootdisks
  and patches to get it to support the 2940U[w].

You need a small patch to use the 2940UW with NetBSD.  Otherwise it seems to
work fine for me.

  Anyways, since they're having trouble supporting it (and
  Windows NT was only slightly more pleased with it --- I had to use the
  vendor supplied disk to get NT to recognise the card --- then the NT
  install died trying to recognise a 3 gig partition (refused to install
  on it))...

We installed NT from cd-rom using the Windows based install.  We had to get
an older Windows driver for the 2940 because the current one doesn't work
with the NT install.  The NT driver itself seems to be ok.