RE: undo-kill-buffer

[Drew Adams]

    Yes, i use C-x k .  Mostly it happens when I am doing some
    development that happens to involve a large number of files.
    Often then I will try to kill buffers I'm no longer working with,
    just to help keep organized or other reasons.  Sometimes I'll
    kill a buffer (file) in this way and a moment later
    realize that there is another change I need to make.

I don't have an answer for your question, but you might want to consider
using C-x 0 more and C-x k less, at least for buffers that you are unsure
you might want to revisit.

Having lots of buffers around (but not displayed) is not necessarily
bothersome or a performance hit. It can be a bit bothersome if you don't
have a good way of filtering them so that you see only the ones you're
interested in as candidates to visit. I tend to have lots of buffers around,
but I use a good tool to filter them as completion candidates to visit.
There are several such buffer-management or buffer-switching tools
available - check Emacs Wiki:
http://www.emacswiki.org/cgi-bin/wiki/CategoryBufferSwitching.

If you are talking about files, then I'm not sure I understand. You can
always revisit a file, creating a new buffer for it. If the file is not in
the current directory, you can get to it quickly using the minibuffer
history (cycling or matching) or the recent files list.

If you are talking about files that you modify without saving, then see
above, about buffers.

I suspect that if you are envisaging being able to undo buffer deletion
among a potentially large number of previously visited files, the management
problem of specifying the buffer whose killing you want to undo might be
bigger than the management problem you tried to solve in the first place by
cleaning out (killing) unneeded buffers.