I just noticed that the Chesley Awards wikipedia article is barely a stub. I've added the categories, following the template of the Hugo Awards wikipedia article, and started to fill in the nominees and winners in those categories. I believe this will make a valuable addition to the ASFA website's listing of the Chesley Awards by year - getting the opportunity to review each category over the years is useful.
If anyone wants to help fill in the gaps, you can use the Best Cover Illustration - Hardcover category as a template.
ASFA does not currently list the awards any further back than 1997. I dropped an email to the ASFA secretary; I'd suppose there might be press releases to reference as well.
UPDATED to add: The Locus Index to Science Fiction Awards Chesleys page lists all the Chesley winners and nominees and can be used as an authoritative reference.
I took the liberty of adding a Chesley Awards category to the wikipedia science fiction project category list, and assessed the main Chesley Awards article page as high priority for the project.
(I've also created pages (where needed) for all of the 2009 Chesley Award winners on the SF Artists wiki and listed the winners on the main page - congrats to all!)