The reference sheets for older, withdrawn thinkpads can be found (as of August 2009) at:
http://www.lenovo.com/psref/psrefs_withdrawn.html

The ftp links and related workarounds at the main thinkpad forum FAQ (viewtopic.php?f=31&t=1689) do not appear to be working.

Ironically, Lenovo has made it easier to get to these old reference sheets by including the link at the bottom their web page for the new sheets. Just go to the thinkpad forum's link for the new sheet:
"1) Can someone tell me __________ about model _________?
NEW - http://www.lenovo.com/psref/

At the very bottom of this page, the link for "Withdrawn Lenovo and IBM product books (back to 1987)" gets you to the old reference sheets.

My apologies if this has already been covered elsewhere, but I could not find it readily.

I recommend that this be made a sticky. there is a large amount of information on this single Lenovo page.

It's already #1 in the FAQ, isn't it?

It sure is! I didn't see it originally, but it's down there - the withdrawn products link.