@Andreas Nurbo - if a theme is released under GPL (which it needs to be, in order to be included on the wordpress.org repository), then credit links are optional, regardless of what the author says. (Where things get knotty is when the designer has used CC-licensed resources in their theme. I don't personally believe it can be legal to relicence other people's content without their say so, but even if it's legal it's certainly unethical.)

However, just because you can do something doesn't mean that you should. Credit links, as well as being a form of thank you to the designer and an acknowledgement that the blogger didn't do everything themselves, are often the only way of telling readers where the theme comes from and where they can download it for themselves. If people are that bothered by crediting theme designers, they can make their own themes. Problem solved.

Historically, plugin developers have been far more likely to be hardcore GPL advocates than theme developers and that's why plugin linkbacks never really gained traction. I would have no problem including plugin-related links on the front end of a site if that was what the developers wanted. They give me a useful plugin, I give them a link in exchange. It seems fair enough to me, though of course there is always going to be a minority of users demanding something for nothing.