Another related question is whether these types of inlinks are against google's quality guidelines - ie, will you get a google penalty if your site has these types of advertisements? My opinion is that it doesn't matter what a lawyer will say about the quality guidelines. What is important is what Matt Cutts says. And Matt Cutts says that you have to think about what your users want - what provides the best experience for the users? Whether google's algorithm or manual penalizers will whack your site for it right away - or whether your readers will just avoid your site and you won't get links - doesn't matter. Your users want to clearly see and know which links you recommend; they don't want to have automatic link insertions that confuse them.