capture buffer to a logfile?

[Mike <mikee@mikee.ath.cx>]

I'm trying to convert some people to emacs and one of the
features they currently use is 'screen captures' to powerpoint.
These screen captures are 99% text. What I'm thinking of
is a small bit of code to capture the current buffer from
the first line on the screen (displayed) to the last
line on the screen (displayed, so it's not the whole
buffer, just what's visible), then copy that hunk to
another buffer that is their predefined (in .emacs?)
log file. First put a timestamp, then the copied lines,
then a few blank lines, then save and let the user
continue editing.

Anyone have this bit of code?

Mike