I found it on other forum, and sharing for IMT Members...
Based on a recently video here, John M recommends making all of our "web design by" credit links "nofollowed".
So what's going on? Is this another attempt by G to try and stem the flow of PR (which by the way we have no idea where site pages stand on PR these days) or is this an attempt to pass on a warning. If it's a warning of sorts, I'll bet there are at least hundreds, and potentially thousands of "designed by" credit links that a designer could accumulate over the years that could come into play.I guess this kind of looks at things like with regards to unnatural links, for example.
And from our point of view, these kind of links, by default, are things where the webmaster doesn't really place the link explicitly on their own.
The rest of the article can be found at SEO RT.