Google Product Forums
Re: Does Google consider affiliate links as "paid links"?
Crawling, indexing & ranking
Thanks Ashley and DigiX, I'll follow this advice. It's a shame nofollow can't be added automatically. The only reason I was keen not to do it was that it adds time to the publishing process, but it's not too difficult to do in Wordpress.
Thanks for the kind comments on my site DigiX :-)