The Advertising Standards Authority in the UK is planning on investigating SEO practices that involve undisclosed purchases of links and other paid-for but undisclosed (to consumer) exchanges. The UK ASA actions have the blessing of Google, who has long discouraged certain approaches to SEO, such as paid links.
The Advertising Standards Authority already investigates Pay Per Click campaigns. Just because the display of natural search results on websites are sparred from the Committee of Advertising Practice, it doesn't mean that agencies and firms that help businesses achieve those results are free from such governance.
Additionally, it should be noted that even links that may not upset Google's standards, may still be a concern for the ASA. There should be a clear difference between regular content and content that has been paid for. The investigations may focus on big brands but will also include regular websites and blogs.