Below is a patch based on Timo Lilja's cool update to gnus-sum.el
posted to gnu.emacs.gnus (in his environment, for 5.8.8; here, updated
for latest Oort) which provides an interesting and extremely useful
thread tree in *Summary*, substituting textual trace lines via %B for
simple space-based indentation via %I.

See a simple text dump of what this provides:
http://www.cs.cmu.edu/~karl/gnus/screenshots/new-thread.text

I've included the small updates needed to the doc string for
gnus-summary-line-format in gnus.el and related commentary in
gnus.texi.

I've tried it out and it works great.  I haven't decided if I want to
use it for the long term, but it's certainly a useful feature that a
lot of people have talked about over the years.  Could the following
patch be applied by one of those with CVS write access?

--karl