AIGA Position on Spec Work
This article is some months old, but still seemed like a good source, knowing there are plenty of agencies/creatives/designers/etc that encounter clients wanting dreadful spec work on a daily basis. We, at the Egotist, don't support the solicitation of designers & creatives and hope you don't either.
Click the image above and here for the article
They even provided a nice sample letter for you.