Lots of excitement these days in German blogtown: whether it's okay to join PR campaigns, to have commercials on one's web site (or blog), or to keep it straight and avoid any commercial purposes.
Just a few thoughts: in the US, where all blogs are originally from, by all means it's been usual to blog for or against something. Websites and blogs dedicated to presidential candidates ("Democratic Girl") are common. It's not easy to say when a blogger keeps authentic and when she's just doing her job, advertizing for this and that. Perhaps this is founded in the opinion making nature of blogs: it's all opinion and that's why we love - or hate - other's blogs and web sites. But when is one opinion more justified than another? What if somebody is paid for having an opinion?
When you start reading a blog and you stick with it for a while you'll get to know the author, his commentators, his friends and foes. The blog becomes part of your daily life and you're becoming part of the blog if you start writing comments. When does this get disturbed? It's not the link to amazon, nor ebay. It's not even the banner, nor the popup (though this can be annoying, but you can do something against this). You'll feel betrayed if you think the author's trying to fool you - then she's losing her credibility.
To fool her readers could hardly be a blogger's target. She'll get response from her commenters and correct the course or she'll ignore the critics and go down. After all, if some of the more-or-less famous bloggers decide to go commercial then this is not an evil thing per se. People's reactions will show if it was a good decision and if the author has lost credibility or has not. If a blogger tries to fool me she has lost me forever. (Yes, it's an emotional thing.)

BTW: mabuse.de always has refused to do advertizing for anything other than for mabuse.de. No, it didn't make us rich (in money). But if we would bother you with pop-ups, banners and all that nasty stuff, we wouldn't get richer, would we?