It doesn't have officially-supported status (yet - Mark has expressed
interest), but the one I'm currently keeping a close eye on is Lubuntu
- Ubuntu with an LXDE desktop.  LXDE is the first environment I've
found that manages to be both lightweight *and* easily understandable
for relatively new users.  Conveniently enough, as of Ubuntu 9.10 all
you need to do to get it is install a minimal Ubuntu system (use the
alternate CD), and 'apt-get install lubuntu-desktop' for a selection
of default packages, or pick and choose your own.  The 10.04 release
should be very solid from the looks of it.

A note about support / repositories:
Remember that Ubuntu does have LTS (Long Term Support) versions, with
the desktop side being supported for 3 years and the underlying and
server stuff for 5 years.
Even after the support period ends, you can still find repositories
for EOL versions at old-releases.ubuntu.com

 - Tony Yarusso