Web surfers have essentially become blind to the standard 468x60 banner ad, and to a lesser degree to other forms of web advertising. Web ads must avoid the stereotypical "ad" look to be effective. So on the one hand, advertisers and designers integrating ads into websites need to be "sneaky", but on the other hand, we should also be ethical. What's the difference between "good sneaky" and "bad sneaky"? Some examples:
Good sneaky: fool people into reading the ads on your site because they think they're content. Caveat: this is only good if the ads are well targetted at your readership--ie. ads for things they're likely to be interested in. In other words, it's okay to make money off of useful links.
Bad sneaky: fool people into clicking the ads on your site because they think they lead to something other than what they really lead to. Obvious example: ads that look like alert popups with "Cancel" buttons--click the "Cancel" button and unexpectedly go to another webpage. Less obvious example: place an ad next to a photo in a way that makes it look like a caption--people interested in the photo might the ad (without reading it) and end up somewhere unexpected. Even worse than the annoyance to the reader is that, with with pay per click ads, it essentially steals from the advertiser. It's as bad as running a bot to generate clicks.
It all boils down to serving the customer. If you're wasting their time by getting them to read things they're not interested in or follow links to pages they're not interested in, you're not serving them. If you're helping them find things that will be useful or interesting to them, you're serving them. Well targetted ads don't need to look like ads. Loosely targeted ads ("this site sponsored by XYZ, inc.", where XYZ, inc. serves the target market for the website) should look like ads. Badly targetted ads shouldn't even exist, but if they do, they should definitely look like ads.