I just got to thinking, how for blogging software themes or forum themes etc, how it is standard practice for part of the copyright agreement that you have to leave a link to the page of the designer on the theme, which pretty much shows up on every single page of your site. Shouldn't google consider that a paid link? as it is a link that is there as 'payment' for the rights to use the copyrighted theme.
in some cases, but I know of many people that throw together a wordpress theme just for the hopes that lots of people will use their theme so they can get lots of backlinks and PR on their home page