Google will indeed require you to mark specific types of links with the nofollow-attribute. These restrictions can be found within the Google webmaster guidelines.
Search engine guidelines require machine-readable disclosure of paid links in the same way that consumers online and offline appreciate disclosure of paid relationships (for example, a full-page newspaper ad may be headed by the word “Advertisement”).– Google Content Guidelines
If one of the following types of links is placed on your own website or linked to another source, it must be marked with the nofollow value:
- Affiliate Link
- Paid Link
- Paid Advertising (as both text or images)
- Link in/from press releases
- Link in/from Advertorials
- Link in an article which has only a minimal topical connection to the target page
- untrustworthy content
- Link in a widget
- Links to pages which only consist of a form (for example “login” or “register here”)
Video explanation by Matt Cutts / Google on the subject
Further information on the subject:
- Google webmaster guidelines on the link attribute rel=”nofollow”
- Google webmaster guidelines on link schemes
