A Google Webmaster Help thread asks a common question in link building. The question is, are "web site designed by X," where "X" is a link to your company, against Google's guidelines?
Both a "level 4" and "top contributor" replied to the thread. One said:
I would think this is a matter of personal preference, and it is quite common in the industry.
The other said:
As far as we know - there is no harm from them. (Hope not - I use them on all my sites ;))
What do you think? Like they said, this is done very often. My company no longer does this, we haven't in years. But we did it a lot before "link building" was so important - go figure.
Forum discussion at Google Webmaster Help.