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Some sites that include affiliate links inform visitors of the fact, in order to assure visitors that there is no conflict of interest. At the opposite end of the spectrum, some sites auto-generate copy for the sole purpose of having somewhere to place affiliate links, which are disguised as useful resources. Most people's opinion of deceptive probably falls somewhere between the two.
Any perceived conflict of interest is the thing to avoid. On some sites, this will be a more sensitive issue than others. See also "Should a Government Site Use AdSense? [webmasterworld.com]". And of course, if you only put the link within your editorial to try to generate money, and not because you think the site is useful for readers, you already have a conflict of interest ;)
If you meant deceptive as in contrary to search engine guidelines, there's a whole other set of questions. Does a search engine like Google consider affiliate links to be paid links? And what level of responsibility should you have in informing search engines about your links?
If you would mention it then there's no conflict of interest.
If you would still go so far as to link to it through a plain-vanilla, non-affiliated link, then there's definitely no conflict of interest.
If you would no longer mention it, then you know your affiliate link is there solely for commercial purposes.
[edited by: Beagle at 1:10 am (utc) on June 28, 2008]