+1  A: 

You can create an append-only column by choosing the Multiple Lines of Text option and selecting "Append Changes to Existing Text". I believe this only applies to lists, not document libraries. Other kinds of lists may restrict this, too.

Alex
You will only see this option if versioning is turned on for the list. If a new version is created without a new comment, it will appear empty unless you're viewing the full history (like in a display form).
dahlbyk
Thanks for your answers. I am using Sharepoint 2003, and it doesn't appear to give me an option to turn on versioning - so the "Append Changes to Existing Text" option does not come up when adding a "Multiple Lines of Text" column. Is turning on versioning not an option in WSS 2.0 (Sharepoint 2003)?
Dan
Your comment on this not being available in Document libraries was the info I needed
pauliephonic
A: 

Not having an append-only field in a document library made me pretty sad. But this template made me happy again: http://www.microsoft.com/downloads/details.aspx?FamilyId=02C4D1B4-7D53-4B72-B577-3DA23F86EC17&displaylang=en

It's the Document Library and Review template and allows each document to have a "Discuss" button and related discussion thread.

Denis